At least in our experience it is very different than the 3 princess meet and greet was in Toontown ( I can't speak as to Main Street because we have not been there since the change). In Toontown, every time we have been there since DD has been born (4 times), it has been the standard 3 Princesses, Cinderella, Belle, and Aurora (except during
MVMCP when Belle goes out to meet in the park in her red dress, and Snow White takes her place in Toontown). If you wanted to see those specific Princesses, you knew when you got in line you would. Plus,
there was a picture of each of them on the sign at the begining of the line telling you who was there for certain, and the CM at the front of the line would also tell you exactly who you were going to meet. (For example at MVMCP the Princes are usually there with them , but when were at MVMCP in 2009, I asked the CM to confirm they were there before we got in the long line, and she said NO, they had things to do and did not happen to be there and we would not see them that night-- DD did not get in line as she had already seen the Princesses- it saved us an hour of standing in line and finding out the Princes had left).
At PFF, the MOST you will get from the CM is a listing of who the possibilities for that day are which is what the CMs were doing this year when we were there. Last year as Mom2rtk mentioned, they would not even give us that information. Last year, we got "You will find out when you get to the front of the line".
For little girls who have specific Princesses they want to meet, it is a mess. If you have a girl who just wants to meet a princess and doesn't care who it is, you will probably be fine. Usually (and I say usually not always), they have at least one Princess- but you might stand in line an hour to get that one Princess with say the Fairy Godmother standing in the spot next to her (not to minimize the Fairy Godmother because we really like her). As you and your DD are leaving, your DD might watch the Fairy Godmother and the one Princess leave, and 3 Princesses take their place. Your DD then really wants to see the Princesses, so you get back in line.
As you get to the front of the line, you watch the 3 Princesses walk off together, and here comes the Fairy Godmother and the 1 Princess you already saw again.