Just saw your question and thought i would add my two cents.
It really depends on what you are going to do with the autographs and how independent your children are. If you plan on taking the autographs out of the book and putting them in a photo album or scrapbook with the pictures of you child and the character (separately) there is no problem with one book just get three signatures from the characters and make sure he doesn't do them back to back.
It you are just keeping it as an autograph book or are planning to get one of those books that you can place the picture right on the facing page... get separate books because as children grow up they feel like it is a part of their childhood memories and will each want a book.
Just some suggestions...