Yes, thanks. Came to the same conclusion. Obviously they cannot be the only three in existence, or at least to the start of the rebellion. Didn't come from nowhere.
A parallel universe?
Yeah, that's why I included "known". I'm sure there are more, or at least were
more. They're very long lived so low birth rates might just be an evolutionary thing, but yeah. There have to be more than 3.
George Lucas has intentionally withheld any background or history on Yoda or others of his race so we're all curious about them! I don't know if he just wanted to keep an ace up his sleeve or maybe just wanted there to be some more mystery in the lore. Whatever Lucas' intentions, John Favreau has played that ace in this series and I think he won the first hand with it. I gotta admit, as soon as I saw the little guy my interest level went up.
The way they're marketing him is pretty brilliant too. Notice there weren't any baby Yoda toys or promotional material with him in it until recently? Word of mouth and memes blew him up on the internet and made him popular overnight, but if you wanted you some baby Yoda for yourself D+ was about the only way to get him.