I have a box with 3 wires coming in, 1 Grnd, 1 120vac, & 1 208 vac (240 vac across the 2) I need to change the plug to a residential plug for a dryer (3 wire - 1 Nutrl, 2 120 vac (240 across the 2).
How do I do this? Is it even possible?

You are in a residence with a 3 phase service feed?

Under current level of the NEC, you must use a 4 wire connection, not a 3 wire.

There must be 2 hot conductors, one neutral and one ground.