Old Man,
M’ikar hadin Ashkenazi Jews only wait an hour (or less). Keeping 6 hours, for Ashkenazim, is a chumra. If it became very inconvenient to keep 6 hours and they wanted to change the minhag, isn’t it reasonable that they would pick 3 hours because of the source in Rabbeinu Yerucham?
In other words, even if Rabbeinu Yerucham is a typo (which I am not convinced of), he could still be the source of the 3 hour minhag.
Alternatively, keeping 6 hours was never an “entrenched minhag” in Germany. They may have kept one hour (or less). Faced with the adoption of 6 hours my other Ashkenazi communities they may have compromised and created 3 hours, once again basing themselves on Rabbeinu Yerucham.