We would write or say "a three-in-one celebration". If a number (more than one) of items are combined into one thing, then that thing is treated as a singular item. I have combined garlic, lemon and dill into a three-in-one sauce.
Scientists have developed a three-in-one blood test.
A three-in-one maternity bag: handbag, removable tote, and a changing
mat in one.
Our male grooming three in one shampoo is suitable for all hair types
and it not only cleans but it conditions hair, face and body.
Most style guides advise writing smaller numbers (e.g. less than 11) as words, thus three-in-one and not 3-in-1.