Depends which Flight Sim software you are going to use.
If it's FSX then you don't need GFX power, you need CPU power. Intel Core i5/i7. NVidia GFX card is best as you can customize the anti aliasing.
If it's X-Plane, then it does use GFX power as well as CPU, but 3 cards is over the top even for this. You can run X-Plane on full with a GeForce 680.