Observant Jews do not eat milk and meat together. But what does “together” mean? There are diverse customs on how long one should wait after eating meat before having milk. One hour, three hours and six hours are the main options.
The tradition is based on a statement of the Talmud quoting Mar Ukva on how he eats meat and cheese (Chullin 105a): “I will not eat them during the same meal, but at another meal I will eat cheese.”
The Dutch custom of waiting one hour reads Mar Ukva as requiring the minimal break between meals — more of a break between snacks, really. The tradition of waiting six hours, on the other hand, interprets “until the next meal” as the next mealtime, which in the Middle Ages was on average six hours after the last one.
It is unclear how the German and English custom of waiting three hours started. Some suggest that in those countries people ate more often, allowing Jews there to read Mar Ukva’s waiting until “another meal” as three hours.