Finding frequency of a three-phase generator from phase voltages
Why don't you just attenuate the voltage on one pair of wires and feed it into an opto-isolator like this: -



This picture was taken from here and the major benefit of using an opto isolator/coupler is that you don't have to have your electronics directly connected to potentially lethal AC voltages. This makes them safe to work on and easier to get working as a prototype.

And if you really want to do it via a bridge rectifier here is a site that has a schematic: -