I'd also enjoy this. I main ADC and when I try to place it in 3's I do see myself getting stomped quite often. The problem with Lightbringer is that it makes you, alone, too viable. If you're tristana you shouldn't be face checking a Darius bush to begin with, your teammate bruiser should be while you wait behind.
I feel like this item would make AD carries alone in 3's too viable
PS. The max attainable attack speed is 2.5, meaning in most situations with a 3 second sight refresh you would forever have sight on your opponent giving them absolutely no strategical advantage over you other than being able to land a gap closer, if melee, and if a gap closer is even present.