Invite your daughter's BFF. Only.
When we have parties, and my kids are friends with the sibling... we invite that sibling. NOT all the siblings.
The parents understand.
Throughout those triplets life... the parents cannot possibly 'expect' that all 3 will be invited everywhere for every occasion... and it then gets expensive for the host.... and the parents, have to understand that.
I may seem cold... but that is reality.
We have friends with triplets and another close friend with 3 kids... I told them, since i know them well, that I cannot pay for all 3 kids... and that my daughter is friends with 1 of them. Not all. It is up to the parent, to teach their kids those lessons too....
It can be a real 'burden' for the host or gift giver... when there are triplets. And it is not always the case where you 'have to' invite all of them.
I have even a few years ago, told the Mom of the triplets we know, that we can no longer exchange Christmas presents... because we simply cannot afford it... and we have our own kids and budgets and grandparents to buy for and think about.
on the invitation... what I do also to limit the attendees and cost for me... is I say on the invitation "your child and 1 adult is invited to... such and such party..." thereby, you delineate the party that you are having and the budget.
all the best,
Susan