This 12th-century conductus, Verbus patris humanatur, is the earliest known preserved example of three-part polyphony.* I really wanted to hear what it sounded like, and didn’t have a piano handy, so I thought I would create a quickly notated example just to give the gist. It’s a thirteen-measure solution of the sort that would have been worked out extemporaneously by ear following organal rules. It features contrary motion, and musica ficta in order to avoid that devilish interval, the tritone (in this case, there are F sharps in the upper part to avoid the B naturals in the tenor).
*According to RT, the example previously thought to hold that honor, Congaudeant catholici from the Codex Calixtinus, was actually two separate solutions written into the same manuscript.