Calculating phase currents of an automotive alternator

A three phase bridge rectifier can easily be configured as a single phase type. What you need to watch out for is: -

1. Voltage rating is sufficient
2. Current rating is sufficient
3. It's fast enough to efficiently handle whatever frequency you are putting its way
4. It can deal with surges into charging the smoothing capacitor involved

As you haven't mentioned any of these you'll have to check your self.

No guarantees as to correctness, but this shows the general method.

1. Find the real and reactive power flowing in each phase.
2. Determine the current flowing in each phase, including the angle between phase current and phase voltage.
3. Add all currents vectorially - real and imaginary components. A diagram helps.

By convention, phase rotation is anti-clockwise. An inductive load will make the current lag behind the voltage. (Remember, inductors resist changes in voltage dv/dt.)

Diagrams below are not to scale.