My DD is also 3 weeks and I find her A time really needs to be no more than an hour, otherwise she is OT and fights sleep. An hour of A time isn't much more than a feed, diaper change, and a little bit of "play time" on the mat before she needs to wind down. If we go beyond this then she is harder to settle and her A time ends up being a few hours.
I also find that at 3 weeks she's coming out of the newborn sleepy phase where they will just nod off whenever and wherever, and I need to make a conscious effort to watch her A times, wind her down, and put her down for a nap rather than just waiting and expecting it to happen in its own, yk?