Why not pull two more of them and leave only one? How many pumpkins do you actually need? And how large?

If they haven't even set fruit yet, they're going to get MUCH bigger before the end.

At the VERY least, I'd love to see ONE develop, but there's 2 buds on 1 of em, and one each on the other 2 plants (respectively). The fruit has YET to set for any of em. I've also buried the majority of the leaf nodes for the main vines.

If I cut down 2 of the 3 of these 5'-6' (so far) vines, should I let the single vine holding 2 buds be my only pumpkin plant to care for..?

5-6 feet is just the beginning when it comes to pumpkins. They can reach 20 feet without breaking a sweat, and they'll put out side branches as well [depending on variety of course].

The advantage of having more vines is to increase the likelihood of having male and female flowers open at the same time, for pollination.

If you just want to grow 1-2 pumpkins, you can wait until one fruit is pollinated and growing well for about a week to make sure it won't abort, then cut off the other two vines. Cut, not pull, since they are planted so close together, it would disturb the roots of the one you want to keep.

You might get more smaller fruits by pruning all the vines back.

It really depends on what your pumpkin goals are.

The goal is to have at least 1 pumpkin be harvested before it starts gettin cold. I doubt we have that many 80+ degree days left in this growing zone.
And here's the thing...there are side vines/branches beginning to sprawl over the collards & watermelon & such..and clearly, we all kno that they don't intend on stoppin anytime soon. Sooo, I'm wondering if I can snip those guys off, but keep any others that aren't strangling my others w/ their tendrils..

Sure. Your pumpkin may not grow as large as it might otherwise, but if that's not your goal, you can go ahead and prune.