Going by what I see on the web, the Tascam US-122MKII has a MIDI IN and a MIDI OUT, so you should be able to connect one of the sound modules to the Tascam, as follows:
Tascam's MIDI OUT port ---> MIDI cable ---> Sound Module 1's MIDI IN port
And you would of course connect the Tascam to the computer as follows:
Tascam's USB port <---> USB 2 cable <---> Computer's USB 2 port
Note, this is assuming that what you're calling "sound modules" are indeed sound modules, meaning they receive MIDI data and generate sounds in accordance with the instrument sounds and notes which are indicated by the MIDI data. If one or more of them are something else-- either a MIDI controller or a MIDI instrument (meaning a device that's both a controller and a sound module in one, such as a typical electronic keyboard)-- then the arrangement will be different than what I've depicted above.
A MIDI controller would be connected to the Tascam as follows:
Tascam's MIDI IN port <--- MIDI cable <--- Controller's MIDI OUT port
And a MIDI instrument would be connected to the Tascam as follows:
Tascam's MIDI IN port <--- MIDI cable <--- Instrument's MIDI OUT port
Tascam's MIDI OUT port ---> MIDI cable ---> Instrument's MIDI IN port
Now, if one of the MIDI thingies is a controller, and another is a sound module, they would be connected to the Tascam as follows:
Tascam's MIDI IN port <--- MIDI cable <--- Controller's MIDI OUT port
Tascam's MIDI OUT port ---> MIDI cable ---> Sound Module's MIDI IN port
If two or more of the MIDI thingies are indeed sound modules, and one or two of them have a MIDI THRU port, you can daisy-chain them together as follows (assuming all three are sound modules and two have MIDI THRU ports):
Tascam's MIDI OUT port ---> MIDI cable ---> Sound Module 1's MIDI IN port
Sound Module 1's MIDI THRU port ---> MIDI cable ---> Sound Module 2's MIDI IN port
Sound Module 2's MIDI THRU port ---> MIDI cable ---> Sound Module 3's MIDI IN port
This would mean that the sound modules would be receiving the same MIDI data as each other, so each sound module would need to be set up to respond to a specific MIDI channel or group of channels which are different than the channel or group of channels that the other two sound modules are set up to respond to.
If you tell us what the three MIDI thingies are, we can look up their specifications and guides on the web and offer better advice.