Hello,
I'm not familiar with the software packages you refer to, but I've no doubt they do what you want in a way not too different from systems I'm familiar with.
First of all, you need some sort of midi output on your PC. You may need input too at some point, but leave that aside for now. This can be a speific midi port (a 5 pin DIN socket) or it might be using USB, which will require specific drivers on the PC end, and USB input on at least the first of the external devices, or some sort of interface between the devices to provide the link.
All my bits are just midi connectors, no USB. Some of the packages you refer to may have USB connectivity built in.
Either way, I believe you'll need to tell the various devices that they are linking via USB rather than midi, which may be the default. Some more modern devices may default to USB, or may be able to tell automatically.
With the midi cables, you would just daisy chain the devices, i.e. PC OUT to keyb IN, Keyb OUT to module 1 IN, Module 1 THRU to module 2 IN. Then, whatever plays on the PC will go to all three units, and depending on channel settings etc (as referred to previously) you could be playing all three units, which should have all their audio outputs connected to mixer so you can fade indiv units in or out as needed. This same connection would allow you to play the keyboard manually, and send part of the midi data to the modules as required.
Again, it may NOT matter at all if all three units are making noise, as you could control what you actually HEAR using the faders on the mixer. This is how My system is set up, I presently have only three of my units connected up, but I have them all playing via the mixer. In effect, all three are responding to the midi data, but I use the mixer to fade in the one I want, and fade out the others.
I assume you can do something similar via USB.
When things are set up, you need to load your midi files into the software, and play them. See what happens. The software should have simple facility to load and display a midi file, allow some editing, and allow playback, although all three progs you mention may be much more complex that needed for just this task, which may make things more complicated.
Geoff