I took up the challenge and had a go at writing my own mensural canon. Not an easy task, and I wouldn't say I am happy with the result just yet. I think it still needs some reworking. I wrote this canon in Musescore, and whilst I originally had decided to have two parts in 3/4, and one part in 2/4, I was not able to figure out a way to do this in Musescore. So in the end I decided to write all three parts in 3/4. This made the second part quite complicated as the ratio of the first and second parts is 3:2. I ended up simplifying some of the rhythms to make it less complex. By the third revision the rhythms of the second part were significantly simpler and slower moving. The third part is more straightforward as it doubles the values of the first part.
I have included 2 drafts plus a final version below.