Is there any problem with the practice of downshifting from D to 3 for engine braking, or staying in 3 then pushing the stick to D to avoid the too-soon upshift to 4 in normal driving?

 

I trained it myself. 2008 Forester

Not sure if you got your question answered, but I don't think there's a huge issue or problem with doing this. I used to do it when my car was auto. It may cause a little more wear & tear due to the higher revs and such, but I can't see it being a huge issue.

None at all, that is what it is there for. D3 is also if you are in a hilly area and the car keeps hunting for the right gear. Also for towing. You are just doing the same thing the transmission does but using the carbon based computer instead of the chip based one.