The shower system I am installing has a 3/4 " thermostatic valve that will "feed" one shower head, one handshower/one rainshower overhead and 3 jets...
So..the plumber ran 3/4 copper pipes to the hot/cold 3/4 thermostatic valve...but now he says he can use 1/2" COMING OUT " to the shower head/handshower/rainshower because he says those will reduce to 1/2 " anyway
Is this correct?
But he says he would run 3/4" for the 3 jets

Yes, the plumber is correct... :)

Here, the drop ear elbow that will meet (behind wall) with the shower arm and shower head do reduce to 1/2" so no need for more than a 1/2" pipe.

The 3/4" was required so that you can run multiple 1/2" lines (shower head, rain shower head), or even the 3/4" jet heads, at the same time and still have enough VOLUME of water for a great shower experience!

Back to you...

Mark