Agree that this is no foul under the new "happy dumping" rule, due to A3 being in the vicinity.
But I don't think the ball first hitting a defender has any bearing. Let's say A3 wasn't in the backfield. say it was just the QB, two offensive guards, and the horde of defenders closing in. If the QB dumped the ball into a defender's thigh, we'd have a grounding foul.
Yes, all become eligible after B's touch. but for the purpose of evading a grounding foul, the eligible receiver has to be in the area when the pass is thrown. In the split-second after a dumped ball grazes a defender's thigh, an offensive lineman being nearby ("Look, Ma, I'm eligible!) doesn't get the qb off the hook.
granted, the new rule gives the QB a ton of leeway, but it can't give THAT MUCH leeway, right?