By curiosity - is there any synth able to do that>
The Prophet 10 rev4 has a 2-voice poly mode (the Prophet 5 r4 might also allow this mode but with 5 voices total you wouldn't get 3 voices in duo poly mode regardless).
Without knowing for sure the sonic characteristic you're referring to when you say "a mess", I'm assuming you're getting clipping... so long story short, during sound design you pretty much need to gain stage each sound depending on how it is intended to be used for best results. I think this is even more true with a 3 oscillator synth assuming you're using all 3 for any given sound.
So you have a mix knob on the Trigon for each oscillator... try turning down each volume knob incrementally in about the same amount, while playing chords with the number of notes you typically want out of that particular sound, and expect that the more notes played will require backing off of gain (whether per oscillator or the master vol of the synth) one way or another.
I'll cite the 3rd Wave as an example of another 3 oscillator synth... the user manual explains it well by saying the total level for all oscillators should not exceed a value of 100 (which is also the same possible max value for each knob on that synth) or you can expect distortion... so that could be 40/40/20 or other combinations that don't exceed 100. On the other hand a *certain* amount can sound nice depending on the sound. But even still if I have everything adjusted perfectly to taste on a 3 note chord then suddenly start playing 5 notes at a time with the same sound, the dynamics have changed and it may need some adjusting for that use case.... even if that's just turning down the master a bit, etc.