I don't like 3-key combinations; two is the max. I've tried using numpad but my hand is almost never on that side of the KB, so I think I'll shift to the left side.
I know that I don't want to override things like Ctrl-F. I'm thinking about using the numbers above QWERTY, and then have some kind of flag that I can set with tilde or scroll lock; when the flag is off I can use the numbers for number input.
I'm sure other people have thought about many more creative ideas.