Generally the sites have a lot of brush with between one and three flat sandy places to pitch your tent -- you are really limited to the sandy places. In the drive-in sites, there is a lot of variation, with some having multiple flat areas, and some having one seriously large flat area. The hike-in ones, as noted, tend to be much smaller.
I suspect you can fit your two smaller tents tightly in one pad, and your larger tent on the other.
I also suspect that, if need be, you can pick up another site on a walk in basis. They don't allow all the sites to be reserved. This late in the season, you'll probably be able to grab another site if you want it. When I was there a couple weeks ago (arriving on a Friday night) in high season, there was at least one site that went vacant. I've often seen that.
