It’s in Yoreh Deah Siman 89, Seif 1. The Shulchan Aruch there says that you have to wait 6 hours. The Rema says that you only have to wait one hour, but some are makpid to wait 6 hours and this is the right thing to do (“??? ???? ?????”).
The Shach and Taz there say that one should really wait 6 hours and not rely on the lenient opinion of 1 hour.
TAZ: ??? ?? ???? ????? ??? ??? ??? ????? ??? ???? ??? ???? ???? ???? ????? ?????? ??? ??? ????? ???? ??? ????
Shach: :?”? ??? ???? ????? …??? ???? ????? ??? ?? ??? ?? ??? ????
The Dutch kehilla has the Minhag of waiting 1 hour as per the Rema. When I learned these halachos, I learned that it is not completely clear where the German minhag of 3 hours comes from. It may be a compromise between the opinion of 1 hour and the opinion of 6 hours.