Hi, glad to see you start posting.
How cold was that basement? I don't use a heat plate myself but read about them a lot on here. From what I've read some heat plates are only good down to 50 degrees Fahrenheit, others supposedly can go lower. My understanding is that the temperature under it isn't important, but the chicks need to be able to touch it with their backs. Where I'm heading is that I'd feel a lot better if someone with experience commented on your plan to raise it with bricks if the chicks need the heat.
My brooder is in the coop. I keep one end warm with a heat lamp but the other end cools off quite a bit. I do not lower the heat under the heat lamp as they age, they get acclimated to cooler temperatures by playing in lower temperature areas. Broody hens can raise chicks in temperatures over 90 Fahrenheit. Your chicks are not going to cook at 90 degrees. I fail to see any benefit in gradually reducing the heat output to wean them off of the heat. They'll do that themselves by not going under it as most of yours are doing now.
When you have a room full of people it is pretty common for some to be too warm, some too cool, and some just right. People are comfortable at different temperatures. Chicks can be the same way. Some like it warmer, some like it cooler. That may explain why one of yours is still going under there. It's probably not necessary for it to live and thrive, but it may just be more comfortable.
So to your basic question, when to take out the heat source. When they stop using it. I don't go by a calendar. I don't believe that at 3 weeks and 2 days every chick needs a certain temperature but at 3 weeks and 3 days they don't. Each chick is different and we all have different set-ups. I try to go by behaviors, mine generally tell me what they want (let alone what they need) by their actions. With that said, I'd be comfortable taking the heat source out at 3 weeks in an ambient temperature of 72 F. But just because I'm comfortable doesn't mean you would be. Let them tell you. It won't hurt anything to leave it in, though it may make cleaning a little harder.