what prophecy is made in Julius Caesar

I really don’t think that what you are asking happens in Act 3. In Act 3 sc 1 Artemidorus and the Soothsayer await Caesar in the street. Artemidorus approaches Caesar with a letter explaining what is about to happen to him. Caesar dismisses it saying matters of the state come before him. The letter, in itself, isn't really a prophecy more than a warning. I think the most famous prophesy comes in Act 2 scene 1 when a soothsayer tells Caesar,

“Beware the ides of March”, the day he will be assassinated by a group of conspirators.