We're running a similar setup on our tandem, except that our inner ring is a 26 and the middle is a 42, then the outer ring is Gates CDX CenterTrack (although we previously also used the CDC rings). We're using single-bike Shimano 105 triple cranks; the 5603 model because the crank arm is a bit wider on the 5703, giving less space for the belt-ring. Our main reason for doing this is that we both like to use 165mm long cranks, and there are no decent-quality tandem cranks available in that length.

We use our tandem mostly for touring and relaxed-paced shorter rides, so we spend most of the time on the 42-tooth ring and in the middle of the 11-32 cassette, and just drop down to the small ring for real climbs, and then try to only use the lower 6 gears. Our rear hub is also a 145 mm, and the chainline looks pretty straight when in the middle ring and on the 5th smallest cog (out of 10). We can pedal up to about 50 kph when we want to (although often we don't bother when going above 40 kph), so the gear range is sufficient for us.

In addition, I've previously run a 46-30 super-compact on a single bike using only the inner and middle rings of a triple crank. The 46 tooth was then a TA Specialites - I believe that 46 is the largest middle ring they make (which is also what this pdf says - apparently they make a 48-tooth inner for a double but not a 48-tooth middle for a triple). Good online sources for TA rings in Europe are xxcycle.com (a French site), who do list the 46-tooth middle, and bike-components.de (German), but they only list up to 44-tooth middles. I also have a crank with 46-29 chairings (a Lightning crank with a 94mm BCD double spider).

With all of these super-comact combo's (26-42, 30-46, 29-46) I've been able to get them to shift OK, but I had to play around with the height of the FD until I found something that worked well (which wasn't necessarily a couple mm above the larger ring as it would normally be), and I found that double FDs worked better than triple FDs. Even so, front shifting is never smooth, and the shift between rings requires definitely 2 and often 3 compensatory rear shifts to keep the cadence somewhat decent; you soon get used to doing this, but you may still not like it even after you're used to it.

I spent a LONG time finding the right chainring spacer combination that would make the 26-42-CDX triple setup work. I used triple chainring bolts and nuts (designed to be used when mounting 3 chaingrings on one set of bolts) and had a whole range of spacer sizes to play around with. Even once you've found a spacing that works then you also need to get the belt-rings perfectly centered on the spider. Even the inner nuts from the triple bolts are not long enough to extend inside the belt-ring, so there is nothing to keep the belt ring very centered. I therefore spent even more time loosening the 5 bolts, moving the ring a little, tightening the bolts and then seeing whether the belt tension was even throughout the crank revolution, and trying to get this right on front and rear cranks.

People have mentioned the increased bending moment on the boom tube due to the exterior position of the belt-rings, but I can't say that I'm very worried. After about 5,000 km of riding (half of that fully loaded with touring gear), the bottom brackets are not showing any undue signs of wear. If we are losing any efficiency, the hopefully it's being made up for due to us having cranks that have a length that we're happy with, and are a bit lighter than most tandem crank offerings.