It depends. How big is the mound? How much room did you allot for the plant to spread? How big are the plants right now?

Personally, I'd wait until the plants got a bit bigger so that you can see if any survive and, if more than one survives, which is the most healthy. Of course, if you gave them enough room, why not let all 3 grow? Mel says watermelon can be trellised, so you could trellis them and see what kind of production you get.

Then again, how many watermelon can you eat in one or two weeks?