Doing this and getting the GTX 970 or simply keeping the GTX 960 would be the best option IMHO. What are the specs of the rig you are building?Sell all three and grab a gtx980 used for about $225 or a gtx970 for about $170.
But if not the 3gb 280 should still be better.
280x. I wouldn't sell old tech to buy other old tech, thats just a waste of time.