Originally Posted by
SkyDog75
Sure, you can use two-out-of-three rings on a triple, replacing the outer ring with a guard -- a "bash guard" in mountain bike parlance. You're right that you'd want to set the limit screw such that your derailleur can't shift up and onto the outer ring. That's a perfectly functional setup. The only oddity that you might notice is that you won't be able to shift into the "3" position on your front (left) shifter
It would be better to set up the shifter so that the 2 and 3 positions correspond to the inner and outer chainrings, with the 1 position leaving the wire slack. The reason for this is that if you forget and try to shift against the stop you risk jamming the shifter ratchet, causing an inability to shift back down without undoing the wire to release the tension or even breaking the mechanism. Having the wire slack in the unused 1 position will do no harm.