When three people all match each other at the same autosomal DNA location we call it Triangulation.
From ISOGG: "If you have at least three people
with a common ancestor matching on the same segment then you
can infer that the segment came from that ancestor. "
Inversely, if three people all match each other on the same segment then you can infer they have a common ancestor.
Blaine Bettinger's new DNA relatedness chart, from his site http://www.thegeneticgenealogist.com/