I just tried sending messages again - this time to the four names listed in my possible 3rd cousins list that do not already match with one of my other branches. Hopefully, this time at least one of them replies. They all have shared matches with each other, so hopefully, are all connected somehow to my unknown grandfather.
I also cancelled my Ancestry membership, which comes due again in September. If I get no replies from any of these four, I do not see the point to continuing, although I may join again if I get a good price, as others have mentioned.