Using diodes for reverse polarity protection
Two diodes will not be enough, the problem is the diode might be able to handle large amounts of current as it can only handle 3A because you'll be running the diode in a forward current mode. You would need to parallel at least 7 diodes (21A of forward current). 

For reverse current protection only one diode is needed.

The other problem is heat, even if you were running 3A, with a 0.5V drop this would be 1.5W of power, the diode would get ~30C above the ambient temperature. The 0.5V drop also a waste of voltage/power. 

I would not suggest using diodes at all for high current applications. I would use a pmosfet. Using a mosfet keeps the resistance low when the mosfet is switched on and the current is going in the right direction. In the reverse, make sure the mosfet Vds voltage is higher than the voltage you will be using in your circuit.


Source: https://hackaday.com/2011/12/06/reverse-voltage-protection-with-a-p-fet/