What I would do (after finding seats that fit 3 across). Keep them all harnessed as long as possible. Then, I'd put the oldest in the center and teach them to buckle themselves (before baby comes). That way, if you use a bucket for the baby, you can open the door, let the oldest climb over to their seat, click the baby bucket in the base (while keeping one hand on the middle child), then walk the middle child around to their seat and buckle them in. Then, you can double-check the oldest either from the side while you are there, or turn around from the front to check.
My 4.5yo has been buckling herself for a while now. She cannot unbuckle so I just reach back to unbuckle first, then walk around to get middle child, hold them while you get baby, and oldest climbs out.