Hey,
If I understand you correctly you have it setup the following way:
Track 1: triggers drums from the MPC on channel 1, midi port A.
Track 2: triggers keys from the Triton on channel 1, midi port B.
Track 3: triggers strings from the Triton on channel 2, midi port B.
However, when you hit play on the MPC, you only hear the drums and the keys, the strings are nowhere to be found?
I'll bet the the Triton is in a mode where it only plays one sound at a time, like a 'single' mode. You'll have to put the Triton into a multi mode so that it can load different sounds into different parts and receive midi on different channels.
In the Triton manual there should be a section on how to control the synth from a computer, it will tell you what mode to put it into to load sounds into the 16 different parts and trigger them via midi.
Take care,
Nick