tribrach
noun
tri·brach ˈtrī-ˌbrak 
: a metrical foot of three short syllables of which two belong to the thesis and one to the arsis
tribrachic
trī-ˈbra-kik
adjective 
Word History
Etymology
Latin tribrachys, from Greek, having three short syllables, from tri- + brachys short — more at brief
First Known Use
1589, in the meaning defined above