Why does my bike left gear shift have more than 3 combinations?
Sounds like a friction front gripshifter with a "Micro-Adjust" shifter.  As mentioned in the comments, that will let you "trim" the shifting a bit.  As you change gears in the rear gears, the chain moves a little side to side, and may rub the front derailleur.  That shifter lets you move the front derailleur cage a little bit to alleviate that chain rub.