I would start with two things.

1. See if the 3rd cousins share a common gg-grandfather (plus or minus a generation)
2. Download his DNA and upload it to GEDMATCH (free) and FTDNA ( a small fee). Ask these potential cousins to also uploiad their DNA to GEDMATCH so you can see exactly where they match yur hgusband. Also there may be matches on these two sites that are not on ancestry.

Then perhaps you might want to do a Y-DNA test at FTDNA and see what you have for matches there.