He hasn't written for three days. What is his silence saying?

Three days is a duration, not a diagnosis of the relationship. Its significance depends on your usual contact and any agreement to check in. After daily conversation, a gap can leave a real question unresolved without revealing its cause.

A message can name the change and ask what happens next. There is no universal day when silence becomes proof that someone does not care; you can still decide how long you want to keep waiting for a conversation.