As per the title, anyone got 3 booster seats across the back of a MK2? 

Can't see it happening.
I've got two, and don't think I'd squeeze a third in.. but most booster seats say not to use them in the centre position anyway.

I doubt it'd work. The ski hatch won't even open past the older child's booster seat in mine. I've seen some companies offer custom made frames to hold three seats where there aren't normally fixings for them but I don't know how good they are regarding regulations and fit.

Update, fitted a pair of rivemove brackets that shift the outer 2 isofix points 7cm out per side, all 3 seats fit a treat now with the middle seat belted in.

Sorry, should say this is in the kodiaq, but the back seat is almost exactly the same size