Wonderful news, congrats. Yes, we've had all combinations of 3 of convertible, infant, and booster seats in the rear of our 2002 RX300 as our 3 kids have grown. I remember when our oldest (now 7) was the only one in the rear-facing seat. Sigh....but I digress...
Toughest part is when you have to do any swapping of seats, but they all fit. We have the booster seats on the ends with the shoulder straps.
Be prepared for scraped knuckles if you have to reconfigure them, then enjoy the ride.