I've always had the capsule in the middle. Mainly coz it fit there at first I think.
DD1 - always in the middle, When DD2 was born she went in the middle & DD1 was behind the passenger seat, then when DS went in the middle DD2 went on the passenger side & DD1 on the drivers side as when on quiet roads I can let her out on the road side as she's a bit older & when on busy roads I get her to climb over. Either over the seat in the middle or over the front & out the passenger door.
These days now that DH isn't around DD1 is usually in the front anyway.