I'd say the LR speakers would be about 6-8 feet apart with the TV setup in between. I do not intend to add rear speakers.
Against your stated set of criteria, I would make the following comments
a) A Pair of DSP5200 set up 7-8 feet apart will sound better IMHO than a Trio of DSP3100 within the same constraints
b) Adding a centre DSP3100hc mounted below your TV will likely only add a little to the overall experience. If your 5.2k pair were further apart than 8ft max, then this may change. (As much as I like Trifield, I do believe that its benefits reduce significantly as you move the Front L&R nearer together. This is not exactly surprising!!)