Luffy's goal is NOT to fight the admirals. He has no plan to beat them. Luffy's goal is to get past them, get Ace, and get the hell away. Gear 2 Luffy can move fast. He only needs a couple seconds to achieve his goal, or even just to distract them for a moment whil WB, Oars, Iva, and Jimbei make their way there.
And if he manages to uncuff Ace, then, even wounded as Ace is, they suddenly got a fire logia on their side. Luffy only needs to dodge and be quick enough for a couple moments to suceed, he doesn't need to be powerful enough to fight three admirals at once.
And actually the three of them being in one spot limits their potetial, they'd just get in each other's way if they all shot off attacks.
Hell, at this point, Luffy could unleash a gear 3 giant fist and just simply destroy the platform, and that in and of itself would cause chaos.
@Don:
you always compare the SN to much with luffy in my eyes…do you think luffy would actually go to this WAR! with his crew when ace wouldn`t be involved?
do you think he would thought about the outcome of this WAR! as something which could detain him from achieving his dream?
He went and fought Arlong and Crocadile simply because they made Nami and Vivi cry. He declared on the goernement because they were taking Robin away. He freed Crocadile from prison to save Ace. Luffy does what he thinks is right, consequences be damned, and he would absolutley go there at that point if he thought he could make any difference, seeing the same things the other SN had seen. Zoro and Sanji would probably insist on it too. (Nami and Usopp would both say "No, we need to stay away from there." but then agree to it when they saw how determined Luffy was to go right a wrong.)