My boys had TTTS, with about a 25-30 percent size discrepancy (skirting close to the maximum size you can have and still be survivable). In addition my donor twin had a cord insertion that was less than ideal, so probably some of that size discrepancy was NOT from TTTS (though some of it was), which is probably why both were miraculously healthier than what was expected for them at birth (esp. for my donor twin).
At birth, my donor/IUGR baby was shocking in appearance to me. Keep in mind the only other baby I'd had was almost 10 lbs, he was 4lbs, and his brother was 6lbs. So while his MZ twin did not at all look like a preemie, DS2 very much DID, and wow. To this day they have kept about a 20 percent size/weight differential, but are otherwise healthy.
This was for twins though. I am not sure if triplets are similar or if the risk increases exponentially. What I have heard is that multiples are under stress earlier though, so they may develop a little faster in utereo. I think if you are keeping a close eye on things, that's all you can do. Are you drinking protein shakes? If not, I'd highly recommend doing it. I don't know if it was the placebo value or what, but it was suggested I do that for the TTTS and it did seem to help both my stress level and the babies'. I have heard from too many other TTTS parents to think it's a fluke.
However, the bright side: 10 years on (well, 10 years in June anyway) my boys are happy, healthy, with little evidence (aside from the size difference which we have been told is likely to even out once they hit puberty) that there ever was a problem. My donor/IUGR twin did lag behind on some developmental milestones, but these days he does as well as or better than his brother at many academic tasks, and is better coordinated/more athletic as well (I wonder if because he qualified for OT/PT services in early intervention and DS1 did not?).