An amphibrachic foot (known as an amphibrach) has a short syllable followed by a long syllable followed by a short syllable (SLS or U/U).
Trimeter is three feet per line.
Here’s a perfect example of amphibrachic trimeter (long syllables in bold, all other are short):
Reclining relaxed in the garden
the cat was ignoring my calling,
indifferently birds kept on chirping,
idyllic conditions for poets.
Amusing deception, cat lazy
and silent just waiting for breakfast.
Indolent or working at trapping
a birdie who thinks that he's sleeping?
I'll leave now before I spoil something.
Lawrence Eberhart, Bird Watching
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet