If you are taking readings from 3 wires to earth, than that would suggest that it is a low voltage 3 phase system, ie. the wires are coming directly from the alternator windings.
Normally these 3 live wires would connect to a rectifier/regulator to give a dc output.
You could use the raw output from an alternator, the trouble with that is that as the engine revs go up and down, so does the voltage and frequency.
If there is a voltage to earth on all 3 wires, then that would be a star configuration. If there is no voltage to earth, then it could be a delta configuration as in the diagram below.