Here's my understanding of cable operation for front derailleurs:

High stop and Low stop set the range within which the derailleur moves. If you were to remove those constraints, cable tension would be the only control over the derailleur location. As such, too little cable tension has the cage too close to your seat tube (being drawn there as a result of spring tension). Too much cable tension pulls the cage too far away from the seat tube. The indexed shifters aren't smart enough to know where the chainrings are, they only measure how much cable is pulled. So it possible that too much cable tension will shift past your middle ring while your shifter still has room to go further. At that point, the high stop won't allow the shifter to draw that much cable, and that's the only thing keeping you from sending your chain sailing off into the night.

There is, however, one more variable than has been mentioned so far. If your front derailleur is mounted by a clamp, rather than bolting onto a braze on, it is possible to adjust shifting by rotating the shifter. Sometimes that's just the tweak that's needed. Good luck in any case.