Where has anyone said they should get all the gifts?
Specific prompts from A's letter go to A. B's prompts go to B. If you use C's prompts, give them to C.
Nobody is obligated to read around the collection if they don't have the time or drive. If you don't gift it to them you can't expect them to see it or read it, which means no comments or kudos from them.
If you're mad that you have never seen C around during the rest of the year, don't treat them and don't use their prompts.
Re: Writing to someone else's letter
Specific prompts from A's letter go to A. B's prompts go to B. If you use C's prompts, give them to C.
Nobody is obligated to read around the collection if they don't have the time or drive. If you don't gift it to them you can't expect them to see it or read it, which means no comments or kudos from them.
If you're mad that you have never seen C around during the rest of the year, don't treat them and don't use their prompts.