Couple of observations:
Kn(X) ? looking at the size of what passes for a ?15mm? figure I can?t see how you could fit 4 Kn(X) on a single base.
My Chariot Miniatures LIR cataphracts fit four on a base, though it's tight.
Three on a base shouldn't be a major problem, except in the circumstances Toby describes. It also means more elements of cataphracts per bag of figures.
Wedge Kn(f) ? these have a deeper base than normal Knights. Unfortunately none of the other DBx based rules systems uses the deeper base for Kn(F).
Do DBMM competition games enforce the above basing conventions? i.e. would there be any problem using 3 figures to a base Kn(X) or wedge Kn(F) mounted as normal cav?
The deeper Kn base matters, because it has other effects, such as how far the element recoils, or measuring distances when forming column from line. However, to maximise flexibility, I'd recommend putting the figures on the more common 40x30mm base, and sitting that on a 40x40mm base for DBMM.
Having said that, my experience of competition organisers is that they're fairly relaxed about troop representation, unless there are egregious inconsistencies. The best thing to do is to talk to them. My bet is that they won't care about 3-to-a-base cataphracts, unless (as Toby describes) your army also has Kn (F) which look similar, and they'll suggest a 40x40mm square of cardboard for the Kn (F) wedges.
