This is a Campagnolo cable guide, it has three channels. The one on the right goes to the RD. The one on the upper left goes to the FD. The one on the lower left is unused on my bike. Where should it lead? I'm just curious.

I just noticed the FD cable is rubbing the tube. Gotta fix that!

If it could serve as a cable housing stop, then I suspect it is for an old school front derailleur which takes a piece of cable housing.

Run your FD cable under the lower left guide and then under the upper left guide to clear your seatube, it uses both guides.
Also, slide the guide down towards the BB.
-Bandera

I've always run my FD cable thru the lower channel and then coming out of the upper channel to the derailleur, and this is the way I have always seen it done... now you have me thinking

That's what I've always done too, FWIW.

Same here. And I often wondered why it was designed the way it is. Anyone know?

I see that it could be done the other way but then why is the "intake" of the long channel so nicely rounded?

Good question Darth. It looks like it could work fine the way you have it routed too. Just one of those little Campy lore mysteries we may never know the answer to.

Maybe the outer lip keeps the cable away from the down tube farther, but I don't know why they'd offer two routing options on the left side and not the drive side.

I have long pondered this , too , and have routed the cable both ways. Both seem to have logical advantages and neither impressed me as the definitive "correct" way for a bare cable.
I like the idea that the lower tab might be the one intended for a housing, but I'm not sure it would afford a stop at the end, which it has to have to work against the stop on the FD.
EDIT: I'm revising my position, and base it on another examination of the campy unit as well as the Zeus version (which some might say faithfully copies the Campy). The small lower lip routes the cable to the same position as the RH tab routes the cable for the RD, so I think this indicates intentionality. The Zeus version is made the same way, even has the little tiny tab on the edge between the lower and upper lip that the cable has to pass over...BUT, the upper lip doesn't have the guide finished at the "leading edge" such that a cable routed just to that lip would stay in place: it would ride up even higher than in the Campy guide.
I think this was intentional design by Zeus to discourage making that "mistake".
Check out the pic from Velobase...it's even more apparent when you have one in hand to compare to the Campy.

I'll have to look at mine, but the lower left channel seems to align with the channel on the right side vertically along the down tube, so both cables run at the same height. If you use it as the OP has it in his pic, the left cable will be a bit higher than the right at the guide.

I restrung it. I had just enough cable left, whew!

The tiny tab is to prevent the loose cable from rubbing on the frame in top gear.

nixing the double post... strange

Ding ding ding, both "channels" are for the front cable to be strung in series. Sliding the clamp toward the BB shell will allow the cable to clear the seat tube. Note the little rectangular foot on the guide is to contact the bottom bracket shell.

Probably. The lug stopped it before it got that low.

I've seen those Campy guides both with and without the "lower" guide on the bike's L side. If the lower guide is to be used with a "stopped" FD and a piece of housing, there should be an opposite stop on the lower guide, as well. I can't tell if there is one or not.

I've used a Campy guide that did not have the lower guide, and my shifter cables routed along the side of the DT all the way down, then up and around to the FD. Using a lower guide would have been an unnecessary change in that routing, it appears.

+1 this. Less friction that way.

I think that lower guide, that rests on the BB shell, is the "correct" version. A guide without that "foot", as I prefer to call it, would confuse people, as to it's positioning. This coming from a guy who's only ever seen one in the last 6 months, & figured it out with NO outside help, LOL. EDIT: Never mind, after I read your post again, mine didn't make all that much sense. (?)

I need one of these things really bad,
one is broken at work, its the bosses bike but he still needs one

This is how I have mine set up, it doesn't rub on the frame on either ring.

best thread ever

That's how the Campy braze-on works, too: