Wayne, I have some information on the three Calebs, and hopefully something to help you find the Rev. War vet.
Your first Caleb I have being the son of John Howland, but that information is a bit up in the air and I haven't looked too hard into it. There are some articles about it in the NGS newsletter back in the 1940s, but the researcher was still changing his mind in the early 1990s; unfortunately, he may be deceased now. Caleb shows up in a "military census" in 1777 for Gloucester, RI - check PERSI for source - so any pension records might be in Rhode Island versus Vermont files. Caleb and Mary had nine children. There could be a Quaker connection here, also. (There were some companies of "fighting Quakers", so that a Quaker veteran wouldn't be impossible.)
Your second Caleb was born in Claredon, Vermont and died 14 December 1859, buried in Sudbury. He married Elizabeth Lindsley and they had seven children, including my ancestor James W. Howland.
The third Caleb was probably a nephew of the second, but I have no other information on him.
If you are a Caleb Howland descendant, I would be interested in exchanging more information (a lot of this is by memory as I am not the best filer).
Randy