In a Trinity, how do you assign a sound (ex. bass)to output
#3 so that it doesn't come out of the stereo mix. The only things I found in the manual were about routing effects...

That's what I would like to know. I'm having the same problem with TR Rack, and I'm beginning to think that it can't be done. Isn't it strange, to say the least, that Korg didn't think of that. The korg 01w which is an older keyboard gives you the possibility to route channels (in sequencer mode ) to differente outputs. I don't know the Trinity but I think it's like a TR Rack with the Keys. Check all the pages in sequencer ( or multi ) mode.
Good luck