One Piece Chapter 545

[Splash!]

There is no doubt that the 3 admirals are the strongest force within the marines. But I wouldn't go as far as saying that a single admiral represents approximately 1/3rd of their strength. Let us not ignore the vice admirals. Yes, the ones during the buster call seemed pretty lousy but there are also people like Garp. Remember that Aokiji used to be a vice admiral for some time, even with his ice logia powers. This suggests that there might be some very powerful vice admirals out there (possible admirals in training).

Then there are people like Smoker, Sentoumaru (dunno if he can be considered a marine) and very special scientist named Vegapunk (his creations must contribute signifcantly to their forces)

Although we have seen the strength of an admiral, we still don't know what some of the other marine elites are capable of. I wouldn't write them off as cannon fodder.

Lastly if the pirates have Whitebeard, the marines have fleet admiral Sengoku.

I honestly don't think Whitebeard has a very good chance of winning. Besides, it would be kind of dumb if the World Government were toppled at this point. It would make the efforts of the revolutionaries pointless.