I thought I'd put together a little howto on how to make this game work in ScummVM:
I assume that you have at least 1100 megabytes of hard disk space free for your game files and for your savegames, and that you already have ScummVM installed.
1) Make a directory to put all your Monkey Island 3 game files in. Mine was /home/aamir/MonkeyIsland3
2) Insert the first CD and copy the 'resource' folder, the 'comi.la0' and the 'comi.la1' files from the CD to the Monkey Island directory you created.
3) Once it has finished copying, eject the first CD and insert the second CD. Copy from it the 'resource' folder, and the 'comi.la2' file to your Monkey Island directory, saying Yes if it asks you to overwrite any files in the 'resource' folder.
4) Eject your CD, and start up ScummVM.
5) Click the 'Add Game' button and navigate to the Monkey Island directory you made and click Choose.
6) Accept the settings and click OK, and just select Curse of Monkey Island in the games list and click start game.
7) Enjoy!