The flooding of those three vaults seems relatively recent. It might have something to do with the attempts to refill the Hoover Dam.
There are notes in the vaults about the flooding though.
Yeah, but do the notes give any time indication? Vault 3 was only recently taken over by fiends in probably the past decade or so from what I gathered. It sounded like Moterhead was leader back then too. And they only opened their vault in the first place due to flooding.
Vault 34, the one the Boomers come from, seems to have flooded sometime after the Boomer exodus and possibly even after they transported the Pulse gun from Nellis. It was already infested with ghouls by the time the flooding started (drowned ghoul).
Vault 11, I don't recall any notes about flooding.
And the Hoover Dam was only refilled I think sometime in the past 7 years. Which seems like a reasonable timeframe for the fiends to have started occupying Vault 3.