Differences are nonexistant to marginal at best,since they're all 780s.
Only difference it the cooling solution they use.Gigabyte uses their Windforce cooler,ASUS uses their DirectCUII cooler while the EVGA uses their ACX cooler.
All three are extremely good and cool the card quite well.
In my opinion,the EVGAs ACX cooler is the best.Cool'n'quiet.
The ACX is followed by the Windforce and the DirectCUII.
But the differences are very VERY little.