Just a thought.....I don't know if 3 x 5200 occupies much more space that 3 x 3100 -- when you consider stands etc.

You don't have to have identical for Trifield to work - it just gets better the closer you get to the same.

If you are quite narrow in siting 2 channels you may find tri doesn't give you much benefit.

....in which case go 2 x 5200 and decide on a centre much later when you've had chance to listen and aclimatise.

Rgds,
Nick