SSM 3. fits TRS upper?
 

And if so, is it a viable alternative to redoing the transom, relocating the engine and converting to Bravo? Or are they too old, scarce and pricey?

No. It doesn't fit TRS anything

Ssm 2 lower uses trs upper. Only thing you can use when swapping to 3's from trs is the trans itself, different tail stock and the boat.

And the 2 is pretty much extinct, right?

Yes

if they can be found, what props fit?

Jamie did you ever sell the #3s you had?

lil red has some #3's in SWAP section

Team Archer on the board had some #2 lowers I think

i have been convinced it isn't enough of a performance improvement to go from something where parts are hard to find(TRS) to something where parts have to be fabricated.