Wouldn't I just keep them in the seat until they reached 65lbs? The Radian has 4 years left I think and the 3 in 1 I'm not sure of the expiry but it was manufactured in 2005.
If your 4 year old is only 33 pounds right now but has a 16 inch torso, I highly doubt he/she will ever make it harnessed to 65 pounds. The 2 65pound seats (Frontier and Nautilus) with the highest harness slots in Canada only have top harness slots of 18.25 inches, so that is about 2-2.5 years of average torso growth. It is very unlikely he/she would double his/her in 2 years. I would guess a similar situation for your 2.5 year old, at only 28 pounds now, ever unlikely that he would make it to 65 pounds in a harnessed car seat because it would be outgrown by weight first.
4 YO still fits fine in the harness and baby just had the straps moved to the top notch and there is more than 1" above head not sure how much I haven't measured to be exact. Baby's seat is approx. 16.5" wide is this narrow? wide?
That I don't know, if that is wide or narrow. But I do believe the Radian is the same width or a little narrower. Which is why I suggested buying a new Radian and sticking with the other 2 of your existing seats.
I can try turning 2.5 YO RF but not sure DH would go for this? Could he get into the seat by himself RF? He is extremely independent and would be really! upset if we had to go back to putting him in the seat. Not a great reason I know but something that needs to be dealt with several times a day.
I was also thinking rear facing for puzzling the car seat in place. Something to remember about where you position the car seat is that all forward facing car seats must be top tethered. He should be able to get himself in the seat, if you put that seat rear facing outboard. But if it wouldn't work for him, then leave him forward facing.
It would be a good idea to give the 3 in 1 to the baby since it will likely be outgrown soon by your 2.5 year old.
If you buy a new Radian it wil have a 65 pound weight limit. If your 4 YO still fits in the harness by height but gets close to 48 pounds, you could give him/her the newer Radian and give the older 48 pound Radian to the middle child.