The short answer is that you need a diode sized for double the highest charge current from the wind generator. As they are charging a battery they will all have a higher voltage than the battery - otherwise no current flows... Depending on the output voltage of each of the generators, the output current will relate to the output voltage, so if one generator has a slightly lower output voltage it will supply slightly less current. Output voltages need to be close, but If they vary by a volt the higher voltage generator will just do a little more of the work.