Does soundforge only support mixing from stereo tracks? That would surprise me. It seems like in most audio editing software, that you should be able to mix from 3 mono tracks with them panned as you indicate to sum to the ultimate stereo mix:
1.wav = mono left
2.wav = mono right
3.wav = mono center
> bus mix to stereo L/R final mix track when bouncing/rendering.
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.
I'm not sure that doubling up the omni track, vs. just adding/summing it from a mono track, is 'better'...it seems to me that would lead to increased summing from the omni source(s) with the main stereo pair, and to not necessarily be all that helpful (unless it just makes things easier with the workflow in soundforge - which I have no idea about since I've never used SF). I use Digital Performer, and find it much easier to fine tune a mix working from mono tracks and panning as called for, especially if I want to make any slight adjustments to one of the tracks. That could also just be, however, something inherent to the workflow in DP and what I've found works best for me.