3 weeks is too early to be sure but I see a couple that could be cockerels, looking at comb and legs. Wait another two or three weeks and post photos of the ones in question showing comb, wattles, legs, and posture. We will probably be able to help you if you need help by then.
Heat is not the only thing that determines how fast they feather out. Some chickens have a fast feathering gene and some have a slow feathering gene. That's the gene that lets you feather sex some crosses if you know which parent has which gene. The diet will affect how fast they feather out also. There are probably some other factors.
How are we supposed to know if your brooder is too warm, you didn't tell us anything about it except that it is in your house and you are using some kind of bulb to provide heat. How big is the brooder and how is the lamp set up? How warm is it in your house? Are you taking the chicks outside to play in colder weather? The more you can tell us about your situation the better we can comment on your specific situation.
The 26th is still 6 days away so that means you have 3 week old chicks in with 1 week old chicks. I've had different broody hens wean their chicks at 3 weeks of age with the overnight lows in the low 70's and those chicks have been fine on their own. One time I had a 2 week old chick kill a sibling and try to kill another, the broody hen just watched. I locked it up all day with the outside temps in the 70's, it made it fine and stopped trying to kill its siblings. They were raised outside by the broody so they were exposed to lower temps which probably helped, but those chicks can handle much cooler temperatures than many people give them credit for.
In my opinion the ideal brooder is big enough so you can heat one spot warm enough and let the far end cool off as much as it will. I brood outside all times of the year. In the winter I've seen ice on the far end of my brooder but the end the chicks were on was toasty. I find that the chicks are great at managing their temperatures themselves straight out of the incubator or from the post office if given a choice. That way I don't have to stress myself out worrying about keeping the entire brooder at a perfect temperature. I give them the option.
For what it's worth, I've had chicks raised in my outside brooder go through nights in the mid 20's F before they were 6 weeks old. Those chicks were acclimated by playing on the cold end of the brooder, they had a diet that was 20% protein to help them start out well, and the unheated coop they were in had great breeze protection down low where they were and great ventilation up high. I think all these factors helped.
I find the best way to determine if the chicks are too warm or too cold is to let the chicks tell you. Don't depend on some stranger over the internet spouting magical numbers, go straight to the ones that really know. If your chicks are too warm they are going to line the walls as far from the heat source as they can get and panting. Panting is how they cool themselves. If they are too cold they will bunch up under the heat. If they are OK they will spread out. It's really not that hard and it's a whole lot more reliable than a stranger over the internet like me. Trust your judgment, you are the one looking at them. And trust them, why would they not tell you the truth?.
My other suggestion is to experiment. When you can observe them, remove the heat and see what they do. Or try a lower wattage bulb. They will tell you if something is wrong.
Some people, especially the first time, need guidelines, that's where a lot of magic numbers come from. II understand that. Those are not absolute laws of nature but are usually pretty safe even if you don't do everything perfectly. I'm not a believer in that 5 degree a week rule because of many things I've seen, but it will keep you safe. Some people tell you to start that at 90, some 95, and some 100. Let's examine that a bit, starting at 90 which is often quoted on here.
0 to 7 days - 0 weeks - 90
8 to 14 days - 1 week - 85
15 to 21 days - 2 weeks - 80
22 to 28 days - 3 weeks - 75
29 to 35 days - 4 weeks - 70.
At three weeks yours are good for 75 F, which I find extremely safe. It's the one-week-olds that might need a bit of heat yet, but maybe you can at least use a smaller bulb.
Heat is not the only thing that determines how fast they feather out. Some chickens have a fast feathering gene and some have a slow feathering gene. That's the gene that lets you feather sex some crosses if you know which parent has which gene. The diet will affect how fast they feather out also. There are probably some other factors.
How are we supposed to know if your brooder is too warm, you didn't tell us anything about it except that it is in your house and you are using some kind of bulb to provide heat. How big is the brooder and how is the lamp set up? How warm is it in your house? Are you taking the chicks outside to play in colder weather? The more you can tell us about your situation the better we can comment on your specific situation.
The 26th is still 6 days away so that means you have 3 week old chicks in with 1 week old chicks. I've had different broody hens wean their chicks at 3 weeks of age with the overnight lows in the low 70's and those chicks have been fine on their own. One time I had a 2 week old chick kill a sibling and try to kill another, the broody hen just watched. I locked it up all day with the outside temps in the 70's, it made it fine and stopped trying to kill its siblings. They were raised outside by the broody so they were exposed to lower temps which probably helped, but those chicks can handle much cooler temperatures than many people give them credit for.
In my opinion the ideal brooder is big enough so you can heat one spot warm enough and let the far end cool off as much as it will. I brood outside all times of the year. In the winter I've seen ice on the far end of my brooder but the end the chicks were on was toasty. I find that the chicks are great at managing their temperatures themselves straight out of the incubator or from the post office if given a choice. That way I don't have to stress myself out worrying about keeping the entire brooder at a perfect temperature. I give them the option.
For what it's worth, I've had chicks raised in my outside brooder go through nights in the mid 20's F before they were 6 weeks old. Those chicks were acclimated by playing on the cold end of the brooder, they had a diet that was 20% protein to help them start out well, and the unheated coop they were in had great breeze protection down low where they were and great ventilation up high. I think all these factors helped.
I find the best way to determine if the chicks are too warm or too cold is to let the chicks tell you. Don't depend on some stranger over the internet spouting magical numbers, go straight to the ones that really know. If your chicks are too warm they are going to line the walls as far from the heat source as they can get and panting. Panting is how they cool themselves. If they are too cold they will bunch up under the heat. If they are OK they will spread out. It's really not that hard and it's a whole lot more reliable than a stranger over the internet like me. Trust your judgment, you are the one looking at them. And trust them, why would they not tell you the truth?.
My other suggestion is to experiment. When you can observe them, remove the heat and see what they do. Or try a lower wattage bulb. They will tell you if something is wrong.
Some people, especially the first time, need guidelines, that's where a lot of magic numbers come from. II understand that. Those are not absolute laws of nature but are usually pretty safe even if you don't do everything perfectly. I'm not a believer in that 5 degree a week rule because of many things I've seen, but it will keep you safe. Some people tell you to start that at 90, some 95, and some 100. Let's examine that a bit, starting at 90 which is often quoted on here.
0 to 7 days - 0 weeks - 90
8 to 14 days - 1 week - 85
15 to 21 days - 2 weeks - 80
22 to 28 days - 3 weeks - 75
29 to 35 days - 4 weeks - 70.
At three weeks yours are good for 75 F, which I find extremely safe. It's the one-week-olds that might need a bit of heat yet, but maybe you can at least use a smaller bulb.
My Black Sex Link, however, is younger (around 2-and-a-half-weeks, as is my Production Red, whose already feathering out better than her). My BSL doesn't really have a tail yet, and her wing feathers don't even reach her rear-area, like my PR's do. And I don't know why she's feathering out so slowly. (She's slightly smaller than the PR, too). 
Even though they're younger than my BSL, Superchick, they're already feathered out a little more than she is. 
I wish you nothing but the best, as always.....and for crying out loud, enjoy every minute of free time you find! Hint: I found that focusing on not needing to go outside when it's 20 below zero to gather eggs, feed, and water was a great way to take the sting out of not having the chickens anymore! 
