@PJ I've never actually worn a garter belt and stockings UNDER pantyhose. I'm not sure I understand the point or the look it would yield when the garter straps appear mashed up against your legs under your pantyhose. The only way I wear that combo is with gartered stockings OVER very sheer nude/tan pantyhose. And that's even exceedingly rarely done by me.
Another thought. Wearing pantyhose OVER gartered stockings might even inhibit the garter from working properly to hold up your stockings. If it interferes with the natural tension that garter clips need to stay engaged with the stockings, you might find them detaching under your pantyhose and causing you issues.