Not sure. I have two car seats and one booster seat in the middle. Its legal and safe. I 3rd car seat would not fit in the middles, hence the booster seat. If it fits and it does not become unclipped then ignore them.
They are talking crap....but thats mothercare all over really. We have three seats in the back, 2 high back boosters and a baby seat for when we have Dnephew. We stop with the police quite often and no one has ever said anything (not dodgy, DH is a mechanic and is a bit of a good samaritan, so always stops if sees a car in need!)
I have a mondeo and we have 2 graco booster seats and a graco base with infant seat in the back. I will say access to the plugs is a little hard, may look into the seat belt extensions you mentioned. I know one of the exceptions on the 3 years plus car seat laws is if 2 car seats are already in use and a third can not be fitted then a child over 3 may travel with out.
Three seats can be tricky but is doable. You must have the seat away from the buckle though. We had a Britax seat - baby one then isofix and two boosters at first. We changed one of our Graco boosters for a britax one (had two Graco at first) and then everything worked ok.
btw, buckle crunch does not just mean that a seat is touching the buckle. it is ok for the seat to touch a buckle... buckle crunch is slightly different, will try and find a link to some pics to explain
see here it's fine for the buckle to rest against the side of the seat, or for another seat to touch it. the problem is when it starts going into the routing "passage" or over the frame of the seat which will then cause stress on the buckle itself
I would take any 'advice' from mothercare on any subject with a huge pinch of salt. My Childs safety - ha wouldn't believe a word they said. I don't know where they 'train' their staff but arses and elbows come to mind.
I have three across the back of a Landcruiser - not a wide back seat compared to size of car as I discovered.
Original combination was isofix base and Maxi-Cosi Cabriofix, Britax forward facing car seat and Maxi-Cosi Robi high back booster, it was the narrowest booster by about 2cm than a Britax one but that 2cm made all the difference.
Now I have 2 in Maxi-Cosi Robi high back boosters and one in a forward facing Britax car seat. I have the two booster seats plugging in next to each other so behind the passenger and in the middle. I have facecloths around the plugs so that they remain facing forward to remove some of the fiddle factor. The is no way one of the booster seats could accidently unplug one of the kids.
We have used the In-car safety centre several times when we have needed new car seat combinations so I am very comfortable that what we are using is safe and legal.
Maybe try Halfords they will try different seats in your car for you.
33kns, i don;t know a HUGE amount about the multimac, but i believe it is a Scandinavian company? Making huge generalisations here of course, but they are v. big on in-car safety over there and that is maybe something in their favour?
plus, it bolts onto your car, that's got to be good as well from a safety point of view?
I don't think I'd have any qualms about using one myself.
where in the country are you? if you can get to Milton Keynes I really can't recommend them enough. there is also an in-car safety place in Rayleigh in Essex. Afaik they don't sell seats, but they have plenty you can try in your car and they're very knowledgeable and can tell you if you can get 3 in there