The simplest version (and an effective one) is Michael Skinner's.
Otherwise, check the Vernon's version.
The problem with straight version is that they are often boring, and if the spectator guess correctly, you're fried. I may recommand a storytelling version. I have a 3 shell game storytelling version in my gambling demo (the Shellman).
I've a 3 domino monte too, closer to the card game.
If you're interested by 3 shell game versions, I recommand also Bob Kohler's. His approach may inspire you for 3 card monte.
If you're interested by my storytelling version of the 3 shell game, I can PM you the routine in English, for your eyes only. The reading may be of some interest.
Just ask.
For another 3 shell game, don't miss
Charlie Frye's:
http://www.youtube.com/watch?v=XY_1812N ... re=related