If the mattress on the floor doesn't work, a couple of other options...
(1) Head-to-toe sleeping might allow you to squeeze three kids into one bed. Tapered or mummy-style bags keep feet out of others' faces. We sleep head-to-toe in the small dinette bed in our Scamp
. My wife was skeptical, but after one night she was surprised at how much roomier it felt.
(2) If you're more ambitious, take out the overhead cabinet over the rear gaucho bed and build an upper bunk (assuming you have the gaucho layout- you didn't say). The bunk was a factory option some, I believe.
A lot depends on how often you expect to need the extra bunk.