It seems to me to that Matthew's account should be understood in light of Mark and Luke, and then in particular to what Peter wrote:
2 Peter 1:16-18, New American Standard Bible (NASB)
16. For we did not follow cleverly devised tales when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty. 17. For when He received honor and glory from God the Father, such an utterance as this was made to Him by the Majestic Glory, “This is My beloved Son with whom I am well-pleased”— 18. and we ourselves heard this utterance made from heaven when we were with Him on the holy mountain.
To me Peter is saying that they are basing what they teach about Jesus on eyewitness testimony concerning what they actually saw and heard (in this case a voice from heaven). Seems obvious to me that they were witnesses of something that actually occured. It is easy to see how they would all three see an event, but what would a vision seen simultaneously by all three be like?
It was important in Peter's mind to have two or three eyewitnesses to establish the truth. Would it bear as much weight to say "although this really didn't happen, we all simultaneously saw the same vision about it"?
Jeremiah wrote:
Wouldn't this question be the same whether it was a vision or actual event? It seems likely that they heard something in the conversations between Jesus, Moses, and Elijah that made it obvious who they were. Not much is revealed in the texts about what was discussed.If matt 17, mk 9, and lk 9 are not describing a vision, then how did Peter, James, and John know who the other two men they saw were?