Front derailleurs are tricky. I have a good procedure for the rear, despite it being more complex than the front, but I don't have a good method for the front.
First thing to check is the limit screw. Shift to 1, and make sure the chain is on the first ring. Check if there is tension on the cable (pull on the exposed part of the cable with your hand). If there is, loosen the low limit stop, which is usually the screw closest to you, often labeled L. If you can't tell which screw it is, just look at it closely and you should be able to tell which screw does what. Loosen the screw until it shifts reliably but not so much that the chain falls off.
If there is no slack in the cable, it may be more difficult. You will probably have to let out some cable tension with the barrel adjuster on the shifter. Turning the barrel as if you are tightening a screw will reduce the tension on the cable. Adjust until it shifts reliably from 2-1. If you hit the limit of the barrel adjuster, turn it back out, loosen the pinch bolt that holds the cable and let a bit through. Maybe 1/8" to 1/4". Now adjust the barrel adjuster until it shifts from 2-1 reliably. The problem here is that it will likely be more difficult to shift up now, since there is more slack in the cable. I don't know of a good way to fix this except that you need to "overshift" every time you shift up. Basically you have to push the shift lever (or turn the knob) past where it clicks and hold it there until the chain catches on the next gear. Then release it. It's kind of a hassle but just about every front derailleur system I've used needs that.
Other, more advanced things to adjust:
-See if the derailleur cage is bent or twisted. Sometimes you can bend it or rotate the entire derailleur to tweak it. I don't recommend it.
-The height is adjustable by loosening the derailleur where it clamps to the tube and sliding it up or down. When in the 3rd gear position I think it's supposed to be a few mm above the 3rd chainring. I don't recommend adjusting this either.
-The cables or housing could be sticky. It's easy to pull the housing out of the frame by creating slack in the cable, then you can slide it on the cable and add lube. The shifter could be sticky, but I don't recommend trying to lube it unless you are sure everything else is good.
I think the standard procedure for adjusting a front derailleur it to set both of the limits then adjust cable tension so that you can shift into and out of 2 from 1 and 3.
Finally, note that cables don't actually stretch. The ferrules compress into the housing, and the cable might crease where it's clamped, and if the cable wears into the lining in the housing that could add slack too. Stretch is just a convenient thing to call it.