Ok this is all about personal interests.. and yours may vary.
I think three nights is enough in Amsterdam.. Ive been 4 times now.. and last two times I had someone who had never been before.. so the things we did were visit Anne Franks house. the Rik museum.. and wandered around and visited the coffee shops ..
When I was younger ( my first two trips) I did the visit the Wooden Shoe factory.. the Tulip gardens, the cheese markets , see the Windmills etc.. that kind of stuff does not interest me anymore ( and frankly some of it didnt interest me then but felt I had to do all the stuff )
Do you mean Brugges.. ? One night .. its cute but thats it for me ( are you into lace and chocolate shops.. ) Sorry I dont know any Burges
Brussels.. its been too many years for me to be any help.. I only recall it was a bit boring for me.. but I was younger at that time and perhaps sought something different.
Venice is magical.. 3 or 4 nights for sure. Florence is nice.. but for me .. 3 nights was sufficent.. but others swear they can enjoy a week there easily.. so thats just personal interests and tastes.
I dont know your age or tastes.. so my advice could be completely unsuitable for you.. if you are a huge history buff.. you will want longer in Florence.. and perhaps be able to find more to interest you in Brussels than I did..
Havent done Milan.