There are also left luggage facilities at the airport or at the central train station (convenient for the TXL bus).
The railway staion that is convient for (next to) the start point for the TXL buses is Zoo, not Central.
If you decide to do a double trip to the airport, it will probably be worth buying an all-day bus ticket rather than three singles. That way you will also be able to take multiple bus and subway rides at no extra cost anywhere in the central zones.