I need do connect 3 dynamos to comman voltage output . here Im using DC motors as dynamos. dynamos output is not much higher, its arround 0.5mv - 2v ,and since dynamo will rotate both side , the output will be AC. i think best way to do this is , convert every single out to DC and connect all in parallel.
* Is this idea practical ?
*what are the other methods i can use ?
*what are the diodes i should use ?

 

You're going to be disappointed when you learn about diode voltage drop. A typical silicon diode will drop 0.6-0.7V (the exact value will depend on current, temperature, etc.). You need two diodes to achieve rectification, so the DC voltage at the output will be 1.4V less than the AC input to the rectifier. That means the rectifier will consume most or all of your low-voltage output. Schottky diodes are often used for this reason and will reduce the drop to maybe 0.4V, total 0.8V.

If you really want to get power out of a low-voltage AC source, you could first use a transformer to increase the voltage. A more efficient approach is to use a dedicated energy harvesting IC. Search on those keywords and you'll find there are many available.