What do you think Legendary will do after Godzilla 3 for the Legendary cinematic universe?

Toho can still make films while Legendary is which is why we have Shin-Gojira coming out but yes, it is likely the license will expire and toho gets exclusive rights again.

I don't recall Toho ever saying Legendary can only make three films. It was simply confirmed that Legendary had "greenlit" Godzilla 2 and 3.

Plans may have changed since then. We don't know if Godzilla 3 is Godzilla vs. Kong or something that will come after yet. Ultimately, I think the Legendary Godzilla series after Godzilla vs. Kong entirely depends on the box office. If it makes a great deal of dough I'm sure Legendary will find a way to continue. If not--Well, we'll see.

Either way I'm not too worried about it. It's becoming increasingly obvious that whether it or not it comes from Hollywood or Toho, Godzilla won't end.