We are a family of 5 and have trouble on very few rides. Most will fit at least 3 across, so we often end up dividing up as 3 and 2. A few rides to consider are:
1) Both Toy Story rides (if everyone wants a gun, someone will have to sit alone
2)EE and RnR have individual seats, 2 across (not sure if your 5 year olds would even be going on these)
3) Goofy's Barnstormer (I think it would be too small for 3, never tried)
4) Indy Speedway - You need to be 52" I believe to drive alone, not sure if your 7 year old would be tall enough, my oldest was then, my 2nd not until age 9
5) Kali River Rapids - You will all be on the same raft, but the seats are in groups of 2.
We have fit 1 adult, 2 children on Dumbo and BTMRR.
We have also fit 2 adults and 1 child fine in one seat on Splash Mountain. (DH is skinny, but I am not and we were fine with DD in between)
I found the CM's at Disney extremely helpful at each ride in letting us know the best way for our family to ride. I was very pleased at how few times someone actually had to sit alone.