Yeah, the iM3 is sweet. It'll ruin you for any other three speed hub, that's f'sure.
Anyway, it's still a three speed. Low gear is cable slack. High gear is cable taught. 2nd gear is half way in between. If you really can't stand the grip shift, get yourself a 9- or 10-gear indexed shifter you do like. Set it up so you can pull the iM3 into high gear in one of the top positions, and then note what index position you're in when the cable goes barely slack and you're in low gear. 2nd will be the index position half way in between.