I know on the PA3x, you can assign separate parts as mono sounds, and that would do what you asked. There is a factory patch that features a 3 layered brass section, but the trumpet is in mono, so that note jumps all over the chord .... you can't control which note it will take, but I suppose it is based on priority - maybe first, or last not played? So - set up a 3 tone section and make each tone a mono sample - a single note will play them all in unison, but a 3 note chord will assign a tone to each of the chord notes. You would give up the ability to play more than 3 notes in the right hand, though.
Not sure I like that idea.
The factory patch with the mono trumpet is kinda cool, but it limits some of my creative energy - too much randomness for my taste.