Bigger the better. Also groups of 3 are not likely to work, especially as I doubt they've been reliably sexed. 10 or 12 in a four foot seems to be a good starter. Having said that I kep 4 in with my mbuna, and that worked well for the year or so I kept them, though I never bred them.
I moved house recently , and gave away all those fish (or swapped them for a load of instant ocean). I don't miss any of the malawis, but I really,really miss the Tropheus duboisi - they really are the best of the lakes cichlids, though I'm not really sure why- character I guess. If I can get them cheap again (I got mine at auction, and suggest you try that too), I will probably try with them again, this time by themseleves.