Let me get this straight: Right now your room has two switches, both controlling the same outlet in a three way fashion. You wish to replace one of these switches with a dual switch giving you a total of 3 switches. Yes?

First of all, make sure the dual switch you bought has a 3-way option.

Second of all, determine if the ceiling fan is being fed from the same junction box as the switches or outlets. You will somehow need to run leads between the switch box and the fan, but depending on how the room is wired, they may already be there.

For more information on ceiling fan wiring: