You should be able to turn them off by zone as WM said.
Assuming you can, yes you can run the unit on a 3100 genset and one AC at a time. You'll still need to monitor your use some, such as microwaving dinner while the AC is running might be pushing it, don't turn the electric WH element on, etc. With moderation though, yes the 3100 will work.
If you've not looked into it yet, check out getting the Honda/Yamaha 2000 watt units. You can parallel them to make 4000 watts, or use just one if you don't need AC. Best part, if weight is a concern, they're 49 pounds each.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10