Triplets are more difficult, but any number of kids can wreck the udder if you don't watch carefully to make sure they're nursing both sides equally. When my does first kid, I bring them onto the stanchion twice/day and check the udder for evenness and for any signs of inflammation, dry teats, etc. With very young kids, they nearly always go for the easiest teat, or sometimes for whichever teat they happened to latch onto the first time and they never bother to look for another one until they're older and hungrier. Because of this, it's very easy for problems to crop up in the first two weeks. Whenever I've got kids refusing one side, I'll tape off the one they prefer and train them to nurse from the other. If they just aren't hungry enough to keep the udder nursed down, I'll milk mama until she's even. That way when the kids' appetites catch up to her production, she's got enough milk for them and she's producing it equally from both sides. If the kids only nurse one side, mama's supply may dry up in the other half and then you've got a problem when they get bigger and hungrier.
Hopefully there's no mastitis. This can cause kids to reject that side. If mastitis caused it to dry up it will need to be treated and hopefully it can heal. Feel for hard lumps in the udder tissue and run a mastitis test. It's possible if you start milking the non-productive side it will start producing again, but it may not even out this year. Good luck!