If you don't want to mark up a good book, type out the five random pages. My preference would be to make a table for the three novels you have chosen, with 5 columns.
Go to the above link and you can figure out from there.
Let's say yellow is for dialogue. Let's also say that for one novel and five pages the word count comes to 1214. Let's further say that there are 422 words for dialogue for this one novel.
422 is what percentage of 1214 Calculate: 34.7611202%