i can do it with my camera, and i did...it's very interesting experiment and more fun and photographic challenge than to play with layers.
But.....i 've a Nikon, so i can't help you with your canon, sorry
it's worth to try!
yes, I know, I had THE greatest fun doing this with the camera, had a good laugh and you learn a lot.
I just thought that maybe there was a easier way..but I dont like nikon, they're too big for my small hands..
Canon has been rather slow off the mark in this area. You can set the camera to take 3 different exposures in succession (Automatic Exposure Bracketing) but not combine them in the camera. But you should be able to do that in Canon DPP if you use that.