Hi,

I don't think that would be a bad idea. My main concern would be the sun situation. These sorts of plants do need a good amount of light, to my understanding. If you don't mind them sprawling, they may just vine out until they're in an area with adequate light.

The cross-pollination would only be an issue if you're planning on saving the seeds from these plants for next year. It wouldn't affect this year's fruit/veggies.

Hope that helps!
Jill

In additiona to the sun concerns, Pumpkins/squash have huge leaves and will smother the watermelons when interplanted. Cucumbers will tolerate more shade than Watermelons or pumpkins and are also more conducive to climbing up a trellis. Cross pollination is not an issue.All three are different species. If you are into seed saving, just don't plant several different cultivars of the same species.