I like to know how to compute for the main breaker and main feeder line for a three phase supply and their branches are just single phase?

1. make a load schedule for a three phase distribution panel
2. distribute the single phase load equally on the phases A,B,C
3. Add the total current for eache phase
4. select the biggest current if the load is not balance. And multiply by 1.25 that will be the feeder size..
5. the biggest current times 2.5 will be the size of your breaker.