Reverse polarity protection with MOSFET: low voltage drop, 1-50V supply
Consider using an LM311 comparator powered from the available +/- supplies and driving an SSR.  

Divide the input voltage down by (say) 11:1 as shown. That gives a 45mV to 4.5V signal 
at the inverting input of U1 for a 0.5 to 50V input. The non-inverting terminal is held at about 23mV, so input voltages greater than +360mV (allowing for 10mV \$V_{OS (MAX)}\$) will turn the SSR on. Zero or negative input voltages leave the SSR off. 

The circuit shown presents a small resistive (11K) load across the input. If that's objectionable, an additional high-impedance buffer could be added ahead of the comparator. 







simulate this circuit &ndash; Schematic created using CircuitLab

A suitable SSR might be the 60V-rated CPC1907B, which will look like a ~0.06 ohm resistor when it is on and is okay for 3A (6A rating) (a bit more resistance worst-case as it warms up). 



The same principle could be used with the comparator driving a heftier SSR, or a mechanical relay, which would allow even lower series resistance. It's also permissible to parallel a few CPC1907Bs with the input LEDs either driven from individual 1K resistors or connected in series with single a lower value series resistor.