A line of verse with three feet is known as trimeter (pronounced
with a short i in the first syllable not "try-meter").
Trimeter on its own is rare in English, although, when alternated
with tetrameter, it makes ballad stanza.
From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.