With an optimistic view on the future, NASA built a Vertical Assembly Building with four bays and three Mobile Launch Platforms. They were later repurposed for the Space Shuttle and other launch vehicles.

Were the there MLPs ever all three in use simultaneously:

• During use for Saturn launches?
• Post-Apollo?

"In use" can mean for example two in the VAB for vehicle assembly and one on the launch pad, but all three should have (part of) a launch vehicle attached to it.