my ipad3 64gb only show me 57 gb...is it correct ??

Yes, that's about right!

Yes, that's correct. You will always have much less space than the advertised maximum. This applies to regular hard drives as well as the flash memory in your iPad.

Here is a long technical explanation as to why:

http://compreviews.about.com/od/storage/a/ActualHDSizes.htm

There is also some space consumed by formatting and iOS.

Yes. Don't forget that the iPad has preinstalled apps on it and the operating system as well. This is no different that the way a computer would show available space on the drive. The OS and any preinstalled applications use some of the space right out of the box.