I am a mom to triplets who are girl, boy, boy and are now 10 years old. I can tell you that one gift is a bad idea. No matter how close the triplets are to each other they are still individuals. If they were just siblings with three different birthdays or if it was three different children from your child's class what would you do? Base that on what you would do for these kids.
As a mom to 4 kids money is tight and the best gifts my kids got over the years were giftcards like to the movies, Chuck E Cheese, Mcdonalds things like that, that are expensive for us to do and a rare treat.
Another idea would be to get them gift cards to Claire's or a place like that where they can pick their own jewelry, nail polish etc.
I know it means a lot to my kids to have their own items since they have to share so much with each other.
Chrissy