Originally Posted by jeffmeh
If your 3 prong outlet has H-H-N (standard 3 prong dryer outlet), then why not just rewire your spa panel temporarily to 3 in, 4 out mode?
In the event that the OP has never been inside a circuit panel see above.
As you mentioned a 3 prong is H-H-N. A 4 prong is H-H-N-G. So he is just missing ground. As seen in the photo above, Neutral and Ground both go to the Neutral Busbar.
While I get not wanting to do a lot of work on a place you will only be at for a short time. It wouldn't take much to run a 4 conductor wire to the panel and add the ground. You already have a breaker there. Or convert it to 3 plug for now. Personally I would like to have a ground being there is liquid, metal and electricity but technically both the Neutral that you have and the ground hook up to the same place. It is just there for safety. So if you are fine with passing on a bit of safety, then it would work.