how do i convince my girlfriend the threesome i want is really for her?

not sure you can

but it's not, it's for both of you. if she can't handle it, she isn't ready.

You can't. Especially since it's not. If you have to try to convince her, it isn't for her. It's for you. Don't shit yourself, pal. It ain't for her.

im sure its for her.