This is old poetry, written in difficult and very old-fashioned English--John Donne, "Go and Catch a Falling Star." and I'm not an expert. The poem is about whether there is such a thing as a faithful woman, or whether finding a good women is as impossible as anything on a list of impossible things. I THINK you are right. He starts by saying "If thou findst one, let mee know," because he'd like to go meet her--then changes his mind. "Never mind, I wouldn't go anyway." It wouldn't be worth the effort, because even if his friend found a faithful woman, women are so fickle that by the time he got there, she would have already been unfaithful with "two or three" other men. It's pretty insulting to women. For anyone else answering this question, the whole poem is at http://www.poetryfoundation.org/poem/173383

I think "Yet doe not" is like; Please tell me when you find one, I'll come and take a look. Oh wait, never mind, I wouldn't come anyway. Why would they not come then? That's the last part; By the time you have written the letter to notify me, before I come, she will have been unfaithful (false) with two or thee men.