:happy-wavehello:
I do 3 across in an '05 Highlander daily! Its the same model as the '06.
First, keep your 4 year old in a harness. It is SO MUCH easier to do a 3 across if you aren't worried about trying to buckle a booster. It's the biggest reason why my booster ready (and has been for 2 years) 6.5 year old is still harnessed. Next, think about a convertible for the new baby. It is still possible to do a 3 across with an infant seat, but oh, so much easier with just a 3rd convertible.
What seats are the 2 and 4 year old currently riding in? Is the 2 year old forward facing or rear facing?
The backseat is surprisingly wide in that car though. You will want to install everything with the seatbelt. Don't use LATCH. Then, put the baby's seat on the driver's side. Think about getting a Radian ifyou don't already have one. They are what makes practically any other seats fit in the car.
The seats I have now are a radian, a Monterey (large booster), and a Frontier 85 (large combination seat).
If you can answer the questions I asked, then we can go from there. it is totally do-able.