Uhh... those are pretty specific questions, and I don't know that anyone here has owned all three cameras to really answer your questions.
Here's what I know, in no particular order:
Elph was a product name they introduced at least 10 years ago, it was their very sleek point&shoot camera. Digital Elph means that it is the digital version thereof. I have one, but it's really just a small camera. The pictures are fine, but I don't try anything creative with it.
I don't know the PowerShot (another name Canon uses for a whole line of cameras.)
I really like the S3 and have thought about buying one. It is a good camera (though not exactly small) and allows you a lot of flexibility, and will follow your skills for quite some time. It will be the closest to an SLR by far.
Regarding those fast shutter speeds, don't worry about them. You'll almost never shoot that fast. A typical shutter speed for an SLR is somewhere between 1/60th of a second to 1/250th of a second. What they are offering (1/3600th of a second) should be extremely rare to use. My extremely fancy and expensive Canon 5D can't shoot that fast - and nor would I ever want to.
IS (Image Stabilization) is a nice thing to have. It will help when shooting in a place where it's a little darker than might be considered optimal.
Have fun. Let us know what you get. Let me know if you get the S3, and what you think of it.