It's just weird to me that Luffy never explained how he came up with Gear 3rd, especially if he explained how he developed Gear 2nd. Assuming he wasn't thinking only about assimilating Soru.
I don't buy that "after he came up with Gear 2nd, he started thinking about more ways to enhance his body".
There just wasn't enough time, since creating such techniques really requires some serious thought put into it. Not that Luffy isn't capable of that, just that there wasn't enough time.
I'll make a guess, and say that you two believe that he probably came up with Gears some time between wearing his blue shorts and changing them with black ones. Like he knew that he will have to eat to recover his strenght. So basically he learned Gears and knew everything about them after such a short time.
Isn't that just a bit of a stretch?
I'm betting everything I have here and say that Gear 3rd is inspired before w7, aokiji, and enies lobby. Crazy, yeah sure. But luffy always admire the giants, I mean before fighting Aokiji, maybe he dint think about becoming stronger. Gear 3rd might be on his mind a long time ago, but he din't want to use it. But what about the name? Well simple, the reason for "Gear..." is because of the train right?
So heres my theory:
Gear 3rd was though up BEFORE gear 2nd, but luffy din't bother to use it until he realizes he have to go against cp9. i mean, he went to a giant island right?
He din't give gear 3rd any name, din't use it cos he din't feel the need for it, heck, maybe he doesn't even know how to use it yet like Haki.
Gear 2nd is developed during the trip from w7 to Enies Lobby, reason simply becos he saw how the cp9 fight and develop the technique based on instincts. I'm guessing luffy never really bothered about Gear 3rd, and he just gave it the name becos he wanted to have another form .
Not to mention there were 2 giants on enies lobby too.
"Then, why din't he use gear 3rd against cp9 the first time?"
Simple, he'll hit everyone there with that damn arm.
So its like this
Luffy realizes that he have gear 3rd-> din't give a damn-> aokiji attacks->cp9 attacks-> got gear 2nd becos he witnessed soru and how cp 9 fights-> develop gear 2nd -> realized that he CAN use gear 3rd because he wanted an alternative fighting style-> named them gear 2nd and gear 3rd.
My crazy theory, oh well.