Speedway will let three fit across - but the problem is that often both kids want to drive. They let us use rider swap in order to get all the kids to ride who wanted.
Disney says age 7 to ride without a parent, so I don't think your 5 year old can ride without an adult. 7 Dwarfs Mine train is 2 across, so you might have to keep swapping out to get that one done. Tell them of your problem to see if they'll let you keep trading out. There's a CM before you get in line so there's someone to ask.
Also keep in mind that not all kids might want to ride too.
A ride like TSM will only have two blasters to shoot with but can fit three. We found that the 2 year old would want to sit in a lap to see better anyway, so it meant I didn't get to shoot though.
*I can't remember how we did Barnstormer when my older two were 3 and 6. Either we did three across or they allowed my 6 year old to not sit next to me, even though he was under the age 7 for riding alone. (Husband doesn't fit on this ride, so he hung out with the baby when we rode it).
