Try this
Insert the TTS-1 ( or any multi timbral synth)
Select the instruments you want to use for each midi channel. Use the GUI and save as a pre set.
Insert 3 MIDI tracks
Assign the outputs off these tracks to the TTs-1
Change the input of each track from OMNI to each channel you need per channel.
Make sure the MIDI input of your Interface is selected in MIDI options.
You may have to mess with the track echo input. It should be highlighted.
Those 3 tracks are ready to record or play the soft synth live now.