DNA donors A and B share a segment that overlaps a segment that B and C share. That situation is called an overlap. If C and A share a segment that overlaps with a segment that A and B share and overlaps with a segment that B and C share, then you have a triangulation. A triangulation indicates that A, B and C have a shared ancestor. If genealogical evidence identifies a shared ancestor among the three donors, the triangulation supports the validity of the genealogy,
Sometimes, even when the three donors are known to have the same ancestor, the DNA for overlapping segments may not all come from that shared ancestor. That doesn't mean you have disproven the genealogy. Keep looking for overlaps and triangulations. Sometimes they form clusters which give even more validity.