We have similar issues. We put a 3rd row seat into a 97 4Runner for a year when #3 was born. We just moved up to the LX470 in September. What we have found is we can get three across- with the baby in the middle and the other two in booster seats on each side. One booster is the base only and the other is still with the back but both use the truck's belts now so there is a little maneuvering to reach them but not bad.
My guess is this may work for you: using infant seats for the first few months for the twins and a regular car seat for the older one (assuming he/she still needs the harness from the car seat and not using a regular buckle yet.) When the twins are big enough for convertable seats and need to be turned around to face foward- keep them in the middle row and fold down the "40" side of the middle row for access into the third row and put the older child back there. You can still reach back to help buckle and even keep the other side of the thrid row in the storage position for more cargo room etc.
We put our older two in the rear- the oldest buckles up the middle one and the baby is in the middle of the 2nd row. Keeps us sane since their noise is that much farther back! The `
