First off, dual graphics only works with a single APU and similar architechture GPU. So that 3 way setup would not work in dual graphics mode.
Next the crossfire with 2 graphics cards (discrete not integrated) 6670 and 7770 would be a bad setup because of the difference in card speeds and would cause stuttering.
If you really want to use dual graphics, the card closest to the integrated chip is the 6670. However, you would get more fps just using the 7770 without enabling dual graphics or trying to crossfire.