Yes in the OP's place I'd drop Amsterdam too.

Hello!
12 days and 3 destinations can be possible if you plan well and book everything before departing. You can begin in London and spend around 3 to 4 days, you can see landmarks, taste some great food (try their chicken masala dish and fish and chips) and in those days while doing the Harry Potter tours, their are also some Alice in Wonderland movie set tours (in case you are interested). Then, go to Paris and visit the Louvré museum, Eiffel Tower, do some food tours to taste a little bit of everything in a single night, and book some walking tours to learn about architecture, Notre Dram cathedral and much more in 3 days. Then visit Amsterdam - you are right in 2.5-3 days you can do a lot / everything there - I've heard that they have a traditional parade or something similar for christmas. so you can consider to stay there for does days. I've never been in Belgium, but since it is very close to Amsterdam, you might finish your trip there. Have you booked any tours yet? I can recommend you some in London and Paris.