A.P.
We've all been told that the middle is the safest place for an infant, and when I only had one child (and 2 kids) and a smaller car, that's just what I did. But now we have 3 and a Sequoia and there was no way I could hoist that infant seat over another car seat to put it in the middle every time we got in the car! My kids are 8, 4 1/2, and 2, and my thinking now is to have the smaller ones who are in the 5-point harnesses with more side impact protection by the windows since they're more secure in their seats than the oldest in just a booster and a seat belt as far as side to side motion if we were to be in a crash. I'm with Lynn M. that your 18 mo should be in the middle, the baby behind the driver, and the 3 yo behind the passenger. Good luck!