How should I tell my girlfriend I want to be anal only ?

[Peachy V]

You're not going to "convince her", if she doesn't like it as much as you and is doing it 3 times a week it sounds like she is already meeting you more than halfway. You can bring it up, maybe work on ways to get her to enjoy it more. But if she doesn't want it you have to respect that.

[Backdoorlover]

I agree with @Peachy V here. There was a time in ly life that 3 times a week would have been a dream come true for me. Back then anal was more like once a month tops.

So as said, see that she enjoys it so much that she asks for it more often. Don’t be selfish or you may loose what you have instead of gaining what you want.