Okay, at that age you can probably let them ride together with you behind them when there are times where you can't all fit.
It seems I may have been wrong about Toy Story. I didn't think you could ride three across because of the lap bars. At any rate, they are probably old enough for you to sit on the other side of the car. They aren't so little that you have to worry about them not sitting or anything like that.
On our last trip my boys were 16 and 12. We still rode 3 to a car in Haunted Mansion. I seem to remember us doing 3 to a seat on Splash, but I can't remember if that was before or after the lap bars. Now they ride in one seat, I ride behind them.
Your kids sound old enough that it probably won't be a huge issue on most rides. If they were like 4 and 6 or something it would be more of a problem. Hopefully the three of you will fit together on the Speedway. Plus, in 9 months you never know how much a little one may grow.