I need to connect an LM119 comparator to my circuit. The question is, should I connect pin 3 (ground pin) to ground? Or has it already been connected to ground?

Thanks!

 

the ground pin is for split supplies, like +/- voltages, if its single supply you can connect the gnd pin to the 0v.

 

Assuming you want the output signal going between the plus supply and 0V, then the ground pin is connected to ground (0V) whether you have a single supply or dual supplies.