I do not know of any online versions of the calculation, but I will be happy to post it right here for you.
In = the square root of I^A+I^B+I^C-(IA*IB)-(IB*IC)-(IC*IA)
However, I have found that for field calculations, by determining the load of the greatest unbalance of any two phase loads will give you a very accurate value. EX: A=30amp, B=40amp, C=50amp. The greatest difference is 50-30= 20amp. If you perform the above calculation, the value comes to around 17.32amp. Not close enough for an exam question, but plenty clos enough for field calcs.
Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter