My personal take is that NASA should keep two: Discovery for KSCVC and Endeavor for JSCVC, while Atlantis can be passed to the Dayton, OH USAF museum.
The Smithsonian already has a real one (albeit not one which has been into space), while the mockup at KSC is a joke, compared to the real deal. NASA cannot afford to be left without any on display. That's almost unnatural.
Those US$90M they plan to grossing by selling them to various parties, will not make any impression on NASA's yearly budget.
So I say: Discovery to KSCVC, Endeavor to JSCVC and Atlantis to USAF.
My 2c,
/Admin