How do I hook a 3 pin connector to a 4 pin?

I would bet that the plug has turn signal/brake lights and running lights, but no ground- at least that's what the colors are. (they did sometimes rely on the ball to ground.
You could try buying a four wire plug for the trailer, hook color to color, adding the white ground to the frame.

Moved from Forum Technical Support

I believe DougE is right on the mark. His advice corresponds with standard trailer wiring color codes shown on the Etrailer web site.

On the surface, it appears that if you plug it into a 4-way on your tow vehicle you SHOULD have Stop, Right Turn and Left Turn (if the wiring is true to standard trailer wiring). In lieu of any other wiring going to the trailer you wouldn't have running lights. Not sure if this is even legal in California. There's an outside chance that the 3rd wire (not the green or the yellow) is the running lights and the ground is through the hitch ball using the frames of the trailer and tow vehicle for a ground path. But if you're plugging into a properly wired 4-way on your tow vehicle that shrouded prong on the left will be plugging into a ground.

.Get an adapter is my suggestion

You should convert the trailer to a standard four wire.

I have been unable to locate an adaptor, google searched but no luck. Don't have a clue on how to convert the camper to four wire, I'll search the net and see what I can find.

thanks, Brian

Three wire as shown would normally have turn signals and brake lights but no taillights.
I would start by using a small jumper wire to see what functions are working on the trailer. Connect jumper to a good battery, use jumper cables to attach battery ground to rig. Then connect positive jumper lead to yellow and then green to see what actually works.
Follow black wire to see if it operates a function or is the ground.
You should convert the trailer to a standard four wire.

Don't know if it would work, but it's not a very sanitary solution. Get an adapter is my suggestion.