Curse of Strahd features a Roc in Tsolenka Pass, which is aimed at 8th level PC's, however it's not intended to be a TPK the Roc just wants something to eat. My players encountered it as they were leaving the Amber Temple, and from what little I remember it was pretty rough in that the Roc is at a massive advantage due to high speed flight, even when your party has spell casters. I think our combat only lasted a round or two, where the players inflicted decent damage but I managed to grab a PC and fly off with him; he was a Warlock, with Armour of Agathys up, so I ruled that when the Roc took enough damage, it dropped the PC and flew away. So the Roc escaped without a meal, and that PC was lost in the snowy slopes (to be found by one of Strahd's minions, lol).
In general, a single flying monster vs a group of PC's, in theory the monster has a massive advantage, but it all depends on exactly what kind of setup the players can contrive to even the odds, because as soon as a few of them get a chance to unload, it's a single monster which easily dies very quickly. Even a few bow shots, at range, can make a big difference, as well as readied melee actions if they get a chance.
For example recently I had a red dragon vs my PC's, their hobgoblin archer allies, and some prisoners the dragon didn't want escaping - the players convinced the dragon to fly into the danger zone, and despite two breath weapons and trying to fly off, they killed it in about two rounds from ranged attacks and a nice sleep spell to finish it off, not even a single PC was dropped. It could have gone the other way, if the dragon hadn't been so annoyed that it made a bad judgement call.
Three PC's vs a Roc, I could see the PC's unloading with ranged attacks and doing a bit of damage for a round or two, taking cover near each other, readying strikes for if the Roc comes in to attack... The Roc will have most of its HP, so will fly in and grab a PC and get hammered in return, before it flies off - can it survive? Probably yes, and can it survive the escape? Probably yes. So most likely, the Roc gets a single PC, but is not stupid enough to try a second time. Maybe the PC's can do enough to stop it escaping with a PC before it gives up trying, maybe the players will avoid combat altogether (if they are mostly melee types, they should try). It's a risk - personally I'd try and make sure there are clear options other than just combat.