MisterE - you can't fire a Contactor on Zero-Cross (well you can but it won't do you much good)... think about it, by the time the contacts close, the mechanical lag involved means zero-cross has long gone... and firing the contactor coil at zero cross does a heap of good... a 10W coil is really important when compared with a few kW of Heater...

Hmmm... since I have several commerical Three-Phase Heating Controllers on the market, I have to tread carefully...

Firstly, you only need to control TWO out of the three phases (although all three should be fused/protected).

The easiest (though not the cheapest) solution is to use a zero-cross SSR in two of the phases. All you do is treat the SSR input as if it was an LED. Switch it on, and it does the rest. You will find that Crydom, Carlo Gavazzi (and others) all do Zero-Cross and non-Zero-Cross versions of all their SSR's. Like I said, chose a Zero-Cross version.

The alternative is to use an opto-isolated "Zero-Cross Triac Driver" to fire your main Triac (which is what I tend to do). An Internet Search will provide you with heaps... MOC3061, MOC3062, TD3043... again treat it as an LED... Don't forget you need one in each of two phases.

NEVER EVER hang your PIC in a non-isolated circuit when playing with three-phase power. Trust me, you will die - and worst of all you then won't be able to come back and tell us, so I won't be able to say "I told you so!".