Is double edged sword worth using on adc?

3% damage ''seems'' like a lot but lets say you're autoing for 500 ( which you wont be for a while )

500 x 3% = 15

So the added damage with Double edged sword is 515.

Lets say you're in lane and auto for 100 which is still a lot ( about b.f sword + )

100 x 3% = 3 so 103 is the added damage

:X Does not seem better than feast imo