zukahn1 

Yesterday the left STX MTB 7x3 speed trigger stopped going into 3rd high on my Raliegh MT-200. So I took everything including the trigger apart checked it perfect condition put it back together and changed the lever angle about 5mm and everything works great. Are mid level 7x3 triggers MTB or Brifters so touchey on the triple that even 5mm turn on the levers through stuff off?

dddd

The normal issue with 10+ year-old Rapidfire Plus front shifters is that suddenly the push to the big ring does not engage the cable spool, so nothing happens.

This is almost always the result of the spring-loaded pawl not reacting fast enough to angage it's catch-point on the toothed wheel that moves with the cable spool.

If this came on recently, test my theory by pushing the "big" lever much more slowly, to see if it then engages the toothed wheel going up to the big ring position. This would indicate thickened lubricant hindering speedy movement of the pawl in response to it's very light spring.
Note that the above scenario is also temperature-dependent. Usually such problems arise following the onset of cooler weather.

Other possibilities include too tight of a cable for the spool to move fully to the third indexed position, or too tight of a hi-limit screw, or some combination of both.
Having a cable housing ferrule slip out of it's stop and then fail to move fully back into it's ferrule, adjuster or cable stop would similarly prevent the third indexed position from being accessed using the lever.

Also, are you sure that the third indexed lever position isn't being mistaken for the second indexed position(?), as would occur if the cable slipped and yielded a considerable amount of it's tension adjustment.

Check also that the main lever isn't obstructed by the clamp of a clamp-on grip, or by the brake lever assembly, seat-dropper lever or etc.

Fix is probably going to be flooding the shifter with a foaming aerosol oil containing solvent.
This is the stuff I use:

Prowler 

+1 on dddd reply. Whenever I encounter questionable trigger shifter or brifters operation (at home or at LBS) I disconnect the cable from the FD or RD, as the case may be. Check the operation using hand tension on the cable only, up at the shifter. Don't get faked out by possible derailleur or cable or housing issues. Flushing the shifter with a solvent may resolve the problem (the "Hail Mary play") but may not. The most reliable solution, barring broken internal parts, is full removal of the clock works and soaking in solvent (OMS) over night. But that is beyond what most folks will do.

Narhay

I had the same issue. I soaked in mineral spirits and used WD-40. My issue was the little pawl on the ratchets wasnt springing back into position so I had to massage it and several other moving parts back and forth until the issue was resolved.

PugRider

I'll add to the "same issue" comments here, and yes, some sort of solvent soak/flush is the answer. But once I finally figured out HOW it works in there (at least on the early ones with two levers stacked on top of each other, early Deore I think was the last one I worked on) it became easier and I didn't need to waste so much WD-40. There's two sets of spring-loaded pawls in there, one handles holding the mechanism after the first click (into the middle ring) and another one down the line holds it for the second and final click (into the big ring). You can take a pick or a small screwdriver and flick the pawls--if they don't freely snap back, then that's your problem. Sometimes they will kinda sorta work--they'll release, but then sort of float back down. It can be deceiving, because it's a controlled return that seems like it's supposed to be that way. But it's the seized-up grease making it that slow, and that's what needs to be hit with penetrant or solvent or whatever works for you. Like any bike part, moving it while hitting it with the spray is always a plus. At any rate, good luck!