Is there a word meaning first in a series of three?

I am especially looking for a prefix that means “the first of three,” but just something close to that would be great. I want to use a prefix to attach to another word to indicate that it is the first one out of three total, specifically. Not just first and certainly not third. Or something that means “one third” or indicates a third of something has passed would be great. I hope there is a word for what I’m looking for.