Your allocation would require 12 slots. There are only 6 available. Remember, you need 2 slots to operate a roundtrip flight...one for takeoff and one for landing. In this case, there are only 3 roundtrips available.
Since Airtran is choosing to operate the flights to Florida instead of the ATL hub, I would allocate them as follows.
2 slots to ATA for an additional flight to MDW: This benefits many communities because of ATA's MDW hub to both small and midsized markets.
2 slots to Midex to MCI: Improves service to MCI and connecting opportunities for Midex.
2 slots to USAirways for DCA-PNS: I'm just biased on this one.
If Airtran operated the flights to the ATL hub and hence increased the network benefits, then I would just give Airtran all six slots.