The main textual source for the minhag of waiting three hours between eating meat and eating milk is Rabeinu Yerucham. However, it appears to many that the "ג" in there is actually a typo and it should be a "ו" as it does not match what he wrote in his other works, nor does it match his cited source (Rashi) for that waiting time.
Given this, my question is - excluding Rb. Yerucham, what is the earliest source that mentions a three hour time period?