Okay, good. Now I understand what you're looking for.
The formula for 3-phase is the same as what you mentioned for single phase, except with 3-phase you multiply by 1.73. It's the 1.73 multiplier that you're looking for.
For 3-phase, the formulas look like this:
For amps: I = P / E X 1.73 (watts divided by volts times 1.73)
For watts: P = E X I X 1.73 (volts times amps times 1.73)
Once again, yes, the formula is the same as for single phase, except for the 1.73 part of the formulas.
I hope this is what you need. Please let me know if it isn't.
EDITED TO ADD: Sorry, I posted the formula incorrectly a few minutes ago, but I have corrected it. Thanks again!