The mapped instrument is what you are looking for. Create one in the environment. Cable it to the soft synth you want to trigger. Use the mapped instrument as your Track Instrument.
By default your TD 3 pads are probably assigned to general midi mapping, as are most of the standard EXS 24 instrument kits. So, th basic pads (kik, snare, tom, cymabls, hi hats) should trigger the correct sounds by default for the most part. But if you want a pad to trigger a different note, go in to the mapped instrument, tap on the pad and see what incoming note it is assigned to, and then just route that to a different note within the mapped instrument. It's fairly straight forward.