Don't tie ground and neutral in a plug or receptacle. This is always wrong. It is dangerous because any current flowing through the neutral will tend to elevate the voltage on all the metallic objects in the circuit.

If the supply receptacle has 4 pins with ground and neutral separate, and you are fixing up an adapter, then it is almost always correct to connect the ground, and to not use the neutral at all. Typical examples would be welders and air compressors.

The one possible exception is if you are trying to use an older electric range or electric dryer. The correct way to fix this is to rewire the appliance to take a 4 wire cord, and separate ground and neutral in the appliance, and get rid of the 3 wire cord. But if you insist on using an adapter, then you would connect the neutral and not use the ground. This is a code violation, so do so with caution.