You can use a deeper cabinet than your wall depth if you use wood trim around the part of the cabinet that sticks out from the wall. If you don’t have wood trim in your bathroom, simply prime and paint the trim to match the wall. A 3” deep cabinet will feel very shallow.

I considered that. The medicine cabinet would sit in a wall perpendicular to a vanity mirror. The trim would be visible from the front of the vanity and be reflected in the vanity mirror. I wasn't sure that would look very good.

What about matching the trim with the color/finish/style of the vanity?

I don’t think it matters. After all, there are surface-mount med cabs that have mirror panels for the edges. You won’t mind seeing wall colored trim pieces 1” out from the wall as much as you will mind a too-shallow med cab.

@margimutch: "What about matching the trim with the color/finish/style of the vanity?"

That's an interesting idea though the vanity is Fresca vanity whose gray oak finish I couldn't recreate. I could paint the trim a similar shade or I could just paint the trim the color of the wall as Nancy suggested.