Personally I wouldn't try to add a second to a single except at the moment of birth - and that's any breed, and including first-timers.
If you do decide to give it a try I suspect you'll need to use an adopter or similar, so she can't see who's suckling until the adoptee smells the same as her own lamb - which will take up to a week.
I presume you're rearing the other two triplets on the bottle - easier on you, the single mum and quite possibly the lamb to just rear the three on the bottle, IMO. Sorry!