I assume you are using a NEMA receptacle 14-50R, (3 pole, 4 wire 250/125V) which is designed for one ground, two hot leads and one common or neutral lead.
The 208/120 3 phase , 4-wire system has three hots, one neutral and no ground. (NEMA 18-50R plug).
You will end up putting 120V on a pin that is designed for the grounded neutral conductor. That could be dangerous to someone in the future. It might be safe, depending on how the plug connected equipment is connected, but it probably isn't.
Best to use the correct plug if you need 3-phase power.
But if you only need single phase power, wire the plug with two hots, a neutral and a ground. The only difference will be the hot pins will supply 208 V instead of 240V. Hot to neutral will still be 120V and the ground will be connected.
If you are using 2 hots, neutral and ground to supply a single phase range or similar appliance (via a NEMA 14-50R or similar) I would and have done that.
If in fact you intend to supply a "120/208 3 phase" (3 hots and a ground) load via such a receptacle, I have several reservations, the most significant- where do you put the 3rd hot? Use what would normally be the neutral terminal for a 120 volt lead? Not me.
There are several other NEMA receptacles that are more suitable, even a twistlock 4 pole 5 wire configuration if you want to run both neutral and ground to your receptacle; NEMA L21-50R and L21-50P respectively.
