3 phase power to supply 3 heaters
I happen to have the manual for small injection molding machine at hand- an old Battenfeld 200kN machine. You should try to get the manual for yours. 

In any case the heater zones are simply wired across different phases in delta configuration. L1-L2, L2-L3, L1-L3. They are single phase heaters and switched independently so this is about as balanced as you can get. The hydraulic power is about double the total heater consumption so momentary imbalances of 1/3 of that is not all that big a deal. 

Try to find out the original heater power ratings- they will have been sized to compensate for the heat generated by shear in the plasticizing screw. 

Using a rotary phase converter for an injection machine is somewhat brave- make sure the idler motor is rated a lot higher than the 3-phase motor it is operating.