Not a question of math. Rather, some issues with the power of observation. The web page has a photo of the L10, for crying out loud...
Not to mention the fact that it isn't "Trans Air," Till recently it
was American Trans Air, but now it's just ATA Airlines.
Of course, I suppose the reporter could have counting the ST6L-73 APU along with those three RB211s for a total of four engines...
Dave