Hi Sarah! It looks like you've got a set of three iambs there. An iamb is a metrical foot containing an unstressed syllable followed by a stressed syllable. If I mark each unstressed syllable with a "U" and each stressed syllable with a "/" and then enclose my individual metrical feet in parentheses, the sentence looks like this:
(U /) (U /) (U /)
He wishes fishes' fins.
Since you have three iambs together, you can call the meter iambic trimeter.
If you'd like to speak more on poetic rhythms, I'd be happy to meet with you for a tutoring session! I have some great worksheets and tools for helping students understand different metrical feet. It's a bit like reading music, and the more you practice, the easier it gets!