How to unlock rotation lock on ipad3?

Is there a lock symbol at the top of the screen next to the battery indicator ? If so, then depending on what you've got Settings > General > Use Side Switch To set to (rotation lock or mute notifications), then you can lock/unlock the rotation by the switch on the right hand side of the iPad, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085

If there isn't a lock symbol then try a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.

The configuration is determined in Settings/General/ Use Side Switch. You can set the side switch for Mute or for Lock Rotation.

Based on what this is set for, you either lock the rotation with the switch or by returning to the Home screen and then double pressing the Home button and swiping to the right.