In poems, verses usually follow a strict structure of accented/ unstressed syllables known as metric. In the case of the trochaic trimeter, this is composed of three (trimeter) trochees, which are the combination of one stressed syllable and one unstressed syllable. This means in total there are 6 syllables and three trochees (poetic feet).
This occurs in the line “Little Lamb, who made thee?" because there are six syllables "Li/ttle/Lamb /who/made/thee?" and each pair of syllables includes a stressed syllable and one unstressed syllable or three trochees Li/ttle/Lamb /who/made/thee? (stressed syllables are in bold)
