Can I combine elements in my three-phase oven and feed them from separate mains?
Wired for Europe it is probably wired for 3-phase "wye" (like a Y).  Each leg of the Y has a triplet of heating elements, with all of them tied together "in the middle" at neutral.  The neutral may not be properly wired back to the neutral wire.   

Where they all join in the middle for neutral, that would have to be eliminated.  Each triplet must return its neutral to the same branch it gets its hot from, and the neutrals cannot mix.  You could keep the triplet of elements together though.  So each triplet would need to be isolated from the other neutrals.

It could be run on less than 3 circuits, but would not get as hot / would take longer to reach operating temperature. 

A commercial machine would be built for this kind of reconfiguration; it is common.  

Your bigger problem is the lack of certification-agency listing on  the machine.  If there's an accident, your insurance company may not pay out, and the authorities could hold you liable.