It actually happened

 

Parshas Hashavua וירא   The רמב"ן quotes the רמב"ם in מורה הנבוכים who holds that the story of אברהם & the 3 מלאכים didn't actually happen but was only a vision אברהם had. אברהם dreamt all the details of the story but none of it actually happened.   The רמב"ן partially agrees with the רמב"ם and holds that whenever we find humans communicating with a מלאך there is no physical מלאך standing in front of the human and talking to him but the communication takes place in a vision.  The רמב"ן however fiercely objects the רמב"ם's Pshat in our Parsha and the other places where the רמב"ם claims it never occurred. The רמב"ן writes, "ואלה דברים סותרים הכתוב, אסור לשומעם, אף כי להאמין בהם". The רמב"ן holds that when the תורה describes the מלאך as an איש- אנשים etc. and there is a story that goes along with it then we must accept and believe all the details that it actually did happen. The ריטב"א  agrees with רמב"ן but defends the רמב"ם against the harsh words of the רמב"ן uses against the רמב"ם. Also the אברבנל seems to side with the רמב"ם. He answers all the קושיות the רמב"ן has on the רמב"ם.