The way it sounds, you have a 50 amp 240 volt breaker, with only three wires (and are missing either a neutral or ground wire at the outlet, depending on what this outlet was powering beforehand). You are going to need to run new wires (at least one) to properly install a 50 amp 120/240 split phase service outlet that you are describing.
Now if you don't want to do this, you can adapt what you have in place. You can remove the 50 amp double pole breaker, and replace it with a single pole 30 amp breaker. You will use one of the hot wires and attach it to the breaker. Take the other unused hot wire and make it either the ground or neutral in your distribution panel. Then take the third wire and make it whichever one you need now (neutral or ground).
You now will connect it as a 30 amp RV outlet below and can use an 50/30 adapter on your RV's 50 amp cord so you can plug in. You will be limited though in what you can run to a total of 30 amps, but this should allow one air conditioner and several other things.
All of this is also explained in the second link I provided above, if you click on the appliance tabs to the left hand side of screen and it explains how to do these conversions.