Hi.
Since it's your first time in Europe, you pretty much cant go wrong with virtually any city. There are so, so many cities that are relatively easy to get to from where ever you are by air, road or rail.

London is huge, but there is a large Steem community here so I'm sure people will be willing to show you around, so give us a shout closer to the time.

Of the 3 cities you mentioned, the only one I've spent time in is Prague, and I can say simply, it's one of the best cities I've ever been to in Europe. Needless to say I HIGHLY recommend that.

Of course Berlin and Vienna are also awesome. These are all very old and historical cities.