Because of pjmlp's hint I just looked into the repositories https://github.com/btreut/a2 and https://github.com/metacore/A2OS, but didn't find a coroutines module. I'm aware that Active Oberon (Patrik Reali, 2004) includes concurrency, but this is a different language than the one used for System 3.
Thanks. I don't know the Ulm compiler, and I unfortunately don't have System 3 (can you please provide a link with the source code and/or a working Linux binary?), but I have the source code of V2 and V4 (both ETH and Linz versions, from 1992 to Linz 1.7) where there are no coroutines.
In case you're interested: I realized that I indeed already have System 3 binaries and source code in my collection; unfortunately the original links at ETH don't work anymore; but I have a physical copy of the book "The Oberon Companion" which includes a CD; I now have uploaded the CD contents to Github; here it is: https://github.com/OberonSystem3/TheOberonCompanionCD.
Note that there is no Coroutines.Mod; I neither found one in the A2 sources.