The Triolet first appeared in France in the late 13th Century. It is a close cousin of rondeaus and rondels, two other French forms where repetition and rhyme are extremely important. Like the other Romance languages, rhymes are much easier to come by in French than in English, but that doesn't mean that we get to miss out on the form.
Triolets take the following form:
The English poet Thomas Hardy wrote many Triolets in his lifetime. One of his most famous is "How Great My Grief."
How Great My Grief
by Thomas Hardy
How great my grief, my joys how few,
Since first it was my fate to know thee!
—Have the slow years not brought to view
How great my grief, my joys how few,
Not memory shaped old times anew,
Nor loving-kindness helped to show thee
How great my grief, my joys how few,
Since first it was my fate to know thee?
This five minute video talks more about the triolet form.