Hello :3 Does anybody know how to make something like this? I’ve been serching Internet about hour and I still don’t know how to make rounded edge like that on the second screenshot. I mean how to make something like that from three verticles shown on the first screenshot. P.S. Sorry for my bad English 
You could bevel a vertex to get the curve.
If you want the original rectangle just duplicate the vertex at the start and fill the rest manually
Thanks, you are livesaver :3
Why do you want to do it that way though?