I don't like any to be perfectly honest.
Brynn Margaret has a great flow, but I dislike Brynn with Johnson as I think it sounds masculine. Even though I like Brynn Margaret Johnson she would ultimately be Brynn Johnson which sounds strange.
Shelby Ann Johnson this sounds very southern and ever so slightly 90s to me. I don't know whether I'd fully commit to this choice.
Tessa Grace, finally I never ever liked Tessa for me she's unattractive. However I like Grace.
I would say the best is Tessa Grace just because she works better with Johnson than the other two combinations.
Changing up the combinations a bit:
Brielle Margaret Johnson
Susannah Ann Johnson
Tallulah Grace Johnson