How to connect a 14-30r outlet with only three wires?
A NEMA 14-30R requires four conductors.  Two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor. 

Both the grounded (neutral), and equipment grounding conductors are connected to ground. But only at the service equipment.  Outside of the service equipment they are separate conductors, and cannot be interchanged or interconnected.