Current capacity of a diode
According to the datasheet, the forward drop is less than a volt at 0.5A, so the power dissipation will be less than 0.5W, but only two will be conducting at once, so only 0.25W each average. 

By the way, 3W is a crazy amount of power dissipation for that package, and cannot practically be realized. If that's really in a datasheet, it's under very unrealistic conditions.