While I was pregnant I signed greeting cards "Love, Dan, Gianna, Baby D & Baylie" (Baylie is the dog).
I wouldn't do a bump-centric Christmas card, I'd find that a tad over the top and sorta cringe-worthy if I received one. It's kind of like those bare bump photos people take that are like boudoir shots for pregnant women. They can be really beautiful but they're not everyone's cup of tea, so they should remain private, IMO.
I like the idea of three stockings. Maybe a cute pose of you and DH hanging the third stocking?