I'm not familiar with Soundforge, but the principle should apply in any software...
I think 1.wav becomes LEFT, 2.wav becomes RIGHT, and copy the omni track 3.wav to both left and right parts of a stereo track, and mix that in like you were doing any other mix.
I think it's simpler to just record 2xstereo and one channel will be flatline, so you copy the omni over to match. I suppose if you were doing it every day this might save some drive space.