Three phase phase-angle control problems

Hello all, I recently completed a stand-alone phase angle controller to control a heating element. My circuit is a "textbook" circuit using an LTV814 for the zero cross detection and a MOC3021 to control a TRIAC. Mains Line connection goes to the TRIAC, then to the resistive element and from the element to Neutral.

For my tests I used an atmel mcu that does the required delaying to control the heating element. While connected to the load and using a panel mounted ammeter as well as an ammeter clamp the circuit seems to work flawlessly and with proper linearity.

When I try replicating the circuit for a three-phase heating element I get some erratic behavior while in the lower settings. My ammeters seem to jump around and on some occasions I can see my lamps flickering! I built three identical circuits, I use a transformer on one of the phases to power the logic part of the circuits and otherwise they are standalone circuits. Each Line (L1-L2-L3) goes to a separate TRIAC, there to a different heating element, and then all of them connect to Neutral. I also use a resistor on the TRIAC to avoid false triggering.

Now, my question is.. am I missing something serious? Unfortunately I don't have a multi-channel oscilloscope or a low-voltage three-phase power supply, but when I tested my circuits on the bench to verify them as single phase (before moving to high voltage) they worked fine.

I've tried all three circuits with all different L1-L2-L3 connections to them and I always get that strange erratic behavior. When using them one by one with any of the mains line they work perfect.

Any ideas on what I'm doing wrong?

Thanks in advance

lots of ideas, carefully draw out your circuit and post it on here, the answers will then be obvious.

As it was meant to be a "quick" proof-of-concept project I don't have anything "nice" ready, would this suffice? the only thing missing are the 100nf caps at the t45s power supply lines.