tribrachia

[trī-brākē-ə]
n.
  1. A condition seen in conjoined twins when the fusion has merged the adjacent arms to form a single one, so that there are only three arms for the two bodies.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.