I have a somewhat similar situation.
Through DNA tests I discovered a 4th cousin (J) who matches to me, my sister, and our second cousin D on a 16.5-cM section of the X chromosome (the match to my sister and me is longer), and has additional matches on one other autosomal chromosome (but not the same segments) with me, second cousin D, and another second cousin M (who is first cousin to D). I am rather confident about the genealogy, and the only identified shared ancestors for J, D, M, and me who could be responsible for these DNA matches are the couple that all four of us identify as 3G grandparents. (We are descended from two daughters of the family, so we cannot tell whether our X chromosome match comes from 3G grandma or 3G grandpa.)
I have chosen in the past to regard this situation as confirmation with DNA. With all these cousins sharing DNA, it sure would be nice to be able to do that.
Footnote: Now there is an additional cousin. This is D1, a 4th cousin 1x removed (i.e., child of a 4th cousin to me), who is descended from another daughter of the 3G grandparent couple (but the rest of his genealogy is not as solidly known) who has matches with all of the rest of us, including an 18.9-cM 3-way match with J and me. Unfortunately, D1 has not replied to my email, so I have not been able to confirm what he knows about his other ancestry, to make sure we are not doubly related.