Manual shifting/d-shifting
You have asked a bit of a loaded question. In factory stock trim the PCM will inhibit downshifts until it is "safe" for the unit to do so. That should tell you something right there.
The older 3 speeds can and will suffer damage if they are abused with high rpm downshifting. Sprags can break, drums can explode and so on. Furthermore, if a 3 speed is shifted into neutral at the end of a very high speed /rpm pass it can explode.
SPecifically, why do you ask? Is there a particular style of racing you are wanting to do?
g
if you're trying to manually shift a factory 4l60e valve body/shifter, you're just asking for trouble. besides, the PCM will be able to shift the car where you want it to be shifted every time, why mess with that???

