Here's why:
* Dactyl: A dactyl is a metrical foot consisting of one stressed syllable followed by two unstressed syllables ( ˈ ˘ ˘ ).
* Trimeter: "Tri" means three, so trimeter indicates that a line of poetry has three metrical feet.
Therefore, dactylic trimeter has three dactyls, resulting in a total of three stressed syllables.