This is is a tough one! It might take a little more diligence on your part to make the new chicks adapt to Mama Heating Pad after 2 weeks under a heat lamp, but I think it can be done. The first thing I think I'd do starts at roosting time. When it gets dark, I'd go out with a dimmed flashlight (I usually rubber band a dark colored washcloth over the lens - gives me enough light to work by but it's not glaring) and tuck all the babies under the pad. Hold your hand in front of it to keep them under. When they start to settle down, you'll hear a soft "churring" sound coming from them. They'll most likely cuddle down all together under there and should go to sleep. In about an hour, if you can do it without disturbing them, check them. In most cases they'll still be under the MHP. If not, you'll have to tuck them in again. Once they figure out that the cave is where the heat is, they'll most likely stay under all night. By the way, if you can turn up the pad a notch, that will help. The older chick will spend more time on top, but the newer ones might appreciate the little extra heat. You can turn it back down in a couple of days.
We had someone on the MHP thread teach her 2 week old chicks to use the pad after a heat lamp, and although they never fully adapted the way new chicks do, they still learned that when they got chilly that's where they could go. Don't be surprised if you find them on top of the pad as much as - if not more than - underneath. By 2 weeks most of my chicks did the same thing. What you have to show them now is where that heat is. I'd keep that set up in the garage if you need to, but remember that then you'll have 3 sets of birds to integrate later - the older girls, the single chick and the new chicks. Should be no issue with the 3 week old and the 2 week old, but the easier you can make it on all of them the better.
Please join us over on the MHP thread so that we don't end up hijacking this thread while we work through this adjustment. Lots of folks over there who have successfully switched chicks over and your experience as you do this might help others in the same situation!