The short answer to this question is "no." However, if you find yourself devoting drastically less space to one or two of the books, you should stop and ask yourself whether you have chosen the correct books and whether you are writing about them in the most interesting and productive way.
If you have a free choice of books, you might choose a theme or character that particularly interests you, then select three books that have a lot to say about this theme, or which have similar characters. If you are assigned three books, however, you will need to perform this exercise backwards and work out the main points of similarity and difference to arrive at your thesis statement. For instance, if you were assigned Charles Dickens's Great Expectations, Anthony Trollope's The Way We Live Now and William Makepeace Thackeray's Vanity Fair, you might conclude (correctly) that social class was a major theme in all of them. Your thesis statement would then probably be to the effect that these novels all address the difficulties and perils of social climbing, or perhaps the relationship between social climbing and money.
These three novels all say so much about the topic that it is very unlikely that you would be tempted to devote a wildly disproportionate amount of space to one or two of them. However, if you kept the same theme and thesis, but substituted George Eliot's Middlemarch for Vanity Fair, you would probably incline to give less space to Middlemarch, simply because its themes do not align so well with the one you have chosen. A split of 40%/30%/30%, therefore, is fine. However, if your split is 80%/10%/10%, this is a clear sign that you should choose different texts or a different thesis.