The basic formula is: 3-phase VD=(1.732×K×I×D)÷CM. Here is a website that explains various ways to determine line load and volatage drop, including the formula method and some examples:
You'll find the formula method discussed almost half-way down the page.
I hope this answers your question, but if you need further help with this please let me know.
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!