The original 1974 tower is the one in the middle of the terminal complex. Until somewhat recently, it remained the only one. Due to increased traffic, airport expansion, cramped space in the existing tower, and distant sight lines, it was decided to split up the operation, and put two towers on the far sides of the two sets of parallel runways.
With the airspace configuration at DFW, this is what made the most sense. The two towers operate independantly of each other (linked by land-lines when the need arises).
I do believe, though, that during the late night, when there is not a great deal of traffic, that the middle tower is used, and the other two are closed.
Tom in NO (at MSY)
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina