Whenever you read one of those threads on an old school forum, or occasionally blog, about how weak dragons are, how many people look at the number appearing section?
Classic has 1-4 dragons in a standard encounter/lair. So does AD&D, except for the Gold who is 1-3, excluding Tiamat and Bahamut, who are unique anyway.
That's an average of 2.5 dragons per encounter, if you roll a straight d4. Even if you make it one adult and two juvenile White Dragons, that could be a pretty tough encounter even for a group at the upper Expert end of the scale, assuming the dragons have room to maneuver.
Granted, I like Frank's Companion Set Large and Huge Dragons, but even the Basic Set ones can be tough if encountered in groups.