I wouldn't wait too long to put the three of them together if you are going to brood them yourself. Most of my chicks are brooder raised, so I have quite a bit of experience in this realm (much more than with a broody hen). I would give the broody one more shot, but this is how I would do it:
Remove her from the nest and put her in a box, cage, whatever you have around and slide the chicks (that are ready) under her. Do this tonight, after dark. Tomorrow morning, get up before dawn (I know...I'm sorry) and put food and water outside of the box/cage. Wait for sunrise, then force the mother out of the pen if she doesn't leave on her own. Make sure the food/water is in a dish small enough for the chicks. If she doesn't start taking care of them at that point, you're sort of stuck brooding them yourself. If you can, see if you can encourage them to eat and drink before you try putting them in with momma. That way, they can at least get food and water....maybe she'll figure out the keeping them warm part. I know that you know if she attacks them then all bets are off and you'll have to brood them.
Your brooder can be almost anything that has high enough sides and enough space for the three chicks (yes, you are going to put all three of them together..LOL). Keep in mind that I have never, ever lost a chick due to being tromped on by others. Use an old heating pad and fold it in half to make a tent. Turn it on medium. Make sure that there is ample space for the chicks to be able to walk in. I have a heating pad specifically for this, you can also put it up into a lean to type arrangement, staple the cloth cover (wire side) to the side of the box/crate. What you are trying to achieve here is a space where they can run in to get warm if they need to, just like they would do with momma. With this set up, you don't need to measure the heat all of the time (I have an ecoglow...same concept). When the chicks feel like they need to be warmer, they will migrate to the lower sides of the heating pad, if they want to be cooler they migrate to the center or out of the heat pad tent altogether. Once they start climbing on top of it to sleep, you can just put it on low and let it collapse under their weight, that is usually at the two week mark or earlier in some cases. I don't do the whole 5 degrees a week thing....the mothers don't do that so why should I? I just give them different levels of heat to choose from and let them choose.
As far as putting the three together, I would probably do this tomorrow assuming that #3 isn't too weak. Too weak is not being able to lift it's head by itself. Any measure of strength above that should be put with the rest. Only thing to keep an eye out for is that because #3 hasn't built up it's strength it may topple over when pushed around by the other two. Just set him/her back up when that happens. Within a few hours he/she will be strong enough to keep up with the other two. There will be some minor pecking, but the longer that they are separated the harder it is for them to bond with each other. I've put newly hatched in with 3 days olds and haven't had a problem yet. I do have to watch them though....the newly hatched are not strong enough to right themselves if they end up on their back.
Good luck!
Let me know how it goes and if you encounter any problems.
I will make a point to put them together as soon as I can. I tried it earlier today with #1 and #2, but #2 trembles so yet that it falls over, and #1 was pecking it too much (although it may have settled if I hadn't intervened). #3 can barely hold its head up yet...I am dropper feeding #2 and #3 almost hourly...which is helping.
This is not a happy hatch...from lackadaisical broody to trembling, shaking, curled toed chicks (especially #2 and #3). I've booted #2 and #3, which has helped. #1 seems to be the sturdiest and strongest, straight toed, and is acts like almost a normal chick now, which is why I tried re-introduction with momma to the sad end of her not taking care of it and then attacking it when she finally roused from her deep trance....I just don't think #3 will be strong enough to be left to the elements again for any length of time.
Sigh: so, I am resigned I will likely have to brood them myself, in the house.
I don't have an extra heating pad...where do you find one that doesn't automatically turn off after an hour? I have a heat lamp over the boxes with the purpose of keeping them at the right temp, and still able to hear each other. I will try to put #1 and #2 in together soon again, otherwise I may attempt #2 and #3 as they are both so wobbly. You are right that I need to integrate them quickly so they will attach and bond together....otherwise I will have to get #1 a companion as it cheeps and cheeps for company.
Ah yes, the joys of chicken keeping.

What are the thoughts on this first time broody? Skewed hatch produced poor mothering behavior? Or are her instincts simply too weak? (She's back to sitting on dud eggs that I placed to keep her parked).
Thanks for input.
LofMc