@21 I believe the main difference with close plantings is that you need to water them more and that, as others have pointed out, you will get powdery mildew faster. The latter I wouldn't worry about; the difference between early powdery mildew and later is only a few days, in my experience (once it starts raining in September, they're doomed).
There's also more of a concern with winter squash of getting the flowers pollinated. Close planting should help (more flowers = more attractive to pollinators), as will planting something like a marigold. But you can also hand-pollinate them.
http://www.gardenguides.com/126589-polli…
Those instructions are a little too genteel for me. I just pull off the males and rub them on the females, like they're having flower sex. In a pinch, I've even cross-bred squash varieties when there were no males blooming, and it worked out okay. Pollinators are likely to do that for you with 3 varieties growing in the same bed.