Hi and welcome to Just answer.
Initially, It depends if the elements are wired star or delta.If star, all three elements are connected together at one end, which may or may not also be connected to neutral. In this case each element is rated for the single phase voltage, whatever that is - it depends where you live and what the service voltage is for the 3 phase. Let's assume it's America and the supply is 110V 3 phase, then each element is rated for 110V and they can be connected in parallel. The heating power will be the same as if it were driven by 3 phase, though the supply current will be triple and you had better check that the wiring and breaker can handle it.If delta, the three elements are connected in a ring or triangle, each to one other and one phase. In this case each element is rated for 1.73 times the single phase voltage. You can still connect them in parallel across one phase, but the power will be reduced to 58% of what it would be using 3 phase. The supply current will be just under twice the current using 3 phase; again, check that the wiring and breaker are man enough for the job.
Hope this information will be helpful.
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