I understand his lung capacity could be greatly increased and all of that, and he could do the balloon attack, the difference being he stays the same basically that way; however, but then, how exactly does air into bones make them stronger/ more forceful/ harder?..
( (more bulletproof)? Actually more bullet proof/ repellant/ reflective/ deflective/ refractive I can somewhat get, especially considering what Oda has said about the bullets shape and materials in the OPU. And I get "manga logic" for suspending belief for the controlling of the path of that air flow, so I leave that alone )
Anyway to a lesser extent there's are these questions:
1. There's the question of bones? As in plural?
2.Luffy's air travels from bones in 1 area to the next? But since joints/ ligaments separate them, don't see that happening. And just as I was about to say I could suspend belief if Oda is saying as Rubber Luffy has 1 interconnected rubber bone, but I realized while typing this Oda states something like 'the power moves from 1 bone to the other, meaning Luffy at least has a few(5 at the least I'd say), making that interconnected bone a lot less possible. Even if Luffy has less need for the stuff(realized might be saying wrong word so using 'stuff' so as not to confuse) separating bones from each other they're made to pretty much fit into each other but not connect, not sure how Luffy different.
3. Luffy's blows air to enter his bone...
... in his thumb? or even the space (ligament area between 1 bone of his thumb and the next)?
4. If air can enter his bones, they aren't airtight?, even with some suspended "manga logic" why wouldn't the air just leak out easily?
There's no need to put your minds on gear 3rd. Oda explained gear second since it's plausible while he never bothered on gear third. It only means that it has no logical explanation whatsoever. If it looks big then it must hurt a lot, that's it.
Anyway, I don't think there's gonna be gear 4th. As we have seen Oda went on a diffirent direction with Luffy's upgrades with his fire like attacks.
The cells in all human bodies have openings, one could argue the air seeps through his bones past the ligaments so on so forth until a big hand is made. I believe it doesn't seep through his skin because he actually has pressure applied from the growing or extended cells.
About the gear third, when one part of him increases, ergo the rest should decrease and what we see is irrevocably a chibi from after some time. This is stopped post time skip, due to the usage of haki I assume.