With an optimistic view on the future, [NASA built a Vertical Assembly Building with four bays and three Mobile Launch Platforms](https://space.stackexchange.com/questions/37888/why-did-the-vertical-vehicle-assembly-building-have-four-doors). They were later [repurposed](https://en.wikipedia.org/wiki/Mobile_launcher_platform) 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.