Both ambient temp and brooder temp are important during the first three weeks. Understand that chicks heat themselves by consuming calories. However, they can lose some of the body heat produced in direct correlation to the ambient temperature as long as all they have is down covering their bodies. The warmer the room is, the slower the chicks will lose body heat, and the cooler the room is, the quicker the chicks will lose body heat. The loss of body heat is greater at night when the chicks are not consuming calories. They will want a heat source to sleep under for this reason.
The bottom line is, they need a local heat source during the first three weeks to rewarm under until feather growth provides a good insulation, around three to four weeks.
The heat source needs to provide a heat "imprint" of between 80F-85F. In some locales, the ambient temp is in the 90s or more. Very little in the way of supplemental heat is requires in these cases. In every instance, the brooder must be large enough for the chicks to get away from any heat source to shed any excess body heat.
This all sounds complicated, but baby chicks in the first two or three weeks need your help in this way to regulate their body temps.