Can you identify this song (canon/round in a minor key)?

There’s a little tune that I know, a 3-part round or canon. It’s simple and hauntingly beautiful. It’s in 6/8, I think, and in a minor key (natural minor/Aeolian mode.)

I first came across it as a vocal warm-up exercise in a choir. Later on, I heard some friends, who were not from that choir, singing it too. My friends didn’t know the name of the piece or where it came from.
Here’s a sound file I made of the round. First the tune is played by itself, and then it’s played as a canon/round.

Any help on finding out the name of the tune or the composer would be appreciated.