Does your three phase generator/alternator have field coils or does it use permanent magnets ?
Alternators in most vehicles use a regulator that controls the voltage in the field coils so they can be relatively low power.
If your generator uses permanent magnets you'll have to try doing it the convensional way.
Heres my suggestion :First you'll need to convert the 3 phase to single phase, this is usually done with a 3 phase to single phase transformer, good luck in finding one of those. Another way is to use three single phase transformers , actually that idea is used in some arc welders, but you'll have to get transformers that are rated for the current of the load you are using. You may have some luck finding a suitable transformer in some discarded power amplifiers or stereos.
Next you'll need a bridge rectifier with proper filtering, again the power handling capability of the bridge is determined by the current draw of your load. Then use a suitable voltage regulator circuit rated for the expected current draw of your load.