Can the 1 year old support his/her head yet ? This determines whether the child can sit in a forward facing seat or still needs to lie flat in a rear facing infant carrier .
I used a Graco rear facing infant carrier which clipped into a base which sat on the seat and could be secured by a lap belt as you would have in the centre position .
All forward-facing seats I have seen require a lap & diagonal belt to secure them so the middle seat would be out for this reason .
After my son outgrew his infant carrier , I bought two Recaro 'Young Sport' seats ( so mum & I could keep one in each car , for maximum flexibility ) and these are very robust , strong and Harris seems to be quite comfortable in it over longish journeys ( Glasgow to Aberdeen & Liverpool being about the longest he has sat in it ) . The seat is very secure when strapped in with the car seat belts as per instructions .
Even in my S Class , I think I'd be doing well to get three child seats in across the back seat ( child seats are bulky ) - so I rather think in something as small as an A Class it will be nigh on impossible .
You'd probably be better looking at an estate car with 7 seats - any E Class for example .