A couple things you might try...
One, with your current setup, in the menus you will find a "delete encoded assets" choice. Use that to clear out any garbage in your project forcing everything to be re-encoded and see if that does anything for you.
Two, if that doesn't work, maybe adding movies to the chapter page is messing your up. What you might do instead is open a new iDVD project, pick your theme, then on the main menu drag and drop in your three movies. Now each movie should get its own chapter page separate from the other movies.
I am not home to try it myself, but if memory serves me correctly, dropping in the first one will create a Play and Scenes button. Dropping in additional ones will give you a folder for those and in that folder you will find the Play and Scenes buttons. I am not sure what happens if you grab all three at once and drag in (do you get three folders or a Play and Scene button for one of them and folders for the other?). Anyway, you are free to rename and cut and paste buttons anywhere in your project. So you can arrange things if you wish.
For example, you might simply create a folder for the first one and then cut and paste the Play and Scenes buttons into that folder so you main menu is simply three folders of your three movies. Going into each will give you the Play and Scenes option.
Alternatively, you can rename your first movie's Play and Scenes buttons to something like "Play Disneyland" and "Disney Scenes". Then go into the folder of movie #2, cut and paste those buttons to the main menu, rename them, delete the now empty folder and repeat for the third movie. You would then have a main menu that might have buttons like so...
Play Disneyland
... Disneyland Scenes
Play Soccer Game
... Soccer Game scenes
Play secret video of roommate
... secret roommate scenes
Each scenes button would take you to the menu of just the scenes from that movie.
Patrick