Changing 3-way ceiling fan/light switch to separate fan & light controls
What you're after requires a whole new wire to the fan:  You currently have a single hot wire going to the fan, that supplies power to both the light and fan.  You want to be able to split that out, so you'll need an additional hot feed, one for the fan, one for the light.  So leave the three way switches as they are and get a qualified electrician to install a new 14/2 conductor from one of the switchboxes up to the light fixture. 

You'll either need to install a double switch or expand the switchbox in order to make room for the third switch.