Jen is right. Just one heat source will be adequate for all three ages, set low enough to produce a temp of around 80-85F.
Try to understand the purpose behind reducing the heat incrementally each week. It's to wean chicks off heat starting at the beginning of their second week as their feathers begin to grow in, so they won't need heat at all by the time they complete growing their feathers.
The only spot in a brooder that needs to be heated is just beneath the heat source. The rest of the brooder is for chicks to self regulate their heat needs and to shed excess heat when they need to do so. So your older chicks may avoid the heated spot all together, while the younger ones use it only occasionally, warming themselves instead by huddling next to the conserved body heat of the chicks with more feathers since they don't lose heat as fast as the younger ones. Look at the older chicks as an alternate heat source for the younger ones, because that's exactly what they're doing.