We actually have a connection
on this one as there are quite a few places that qualify.
The Historic Centre of Brugge actually contains three sites, as the Begijnhof is also in the inscribed area. I think it may be the only place that you can see three world heritage sites from the one point inside a World Heritage Site. (either on top of the Belfry or in the centre of the garden at the Begijnhoof)
I think there will be a few places outside a WHS that you can see three inscribed sites though. (A few taller buildings in central London will give you a view of Westminster, Tower of London and Greenwich, I doubt any would stretch to Kew as well).