A tribrach is a metrical foot used in formal poetry. It consists of three short syllables.
The existence of the tribrach has been contested by some writers and it has no entry in the OED but does appear, primarily as a musical form, in some American dictionaries, such as Virginia Tech Multimedia Music Dictionary
I have also seen it referred to a tripod as below.
A tribrach can also mean a three-footed device (see theodolite).