What is a plural for 3 day?

The plurality applies to the subject or object. So if you are simply refering to multiple days, it would be "This project will take three days" day or days is the subject. If you are refering to something, to which the 3-days is an adjective, then the plural/singular will still apply to the main subject e.g. "This is a 3-day project" or "I have two 3-day projects back to back."