Keyboard shortcut for prev/next sheet in Numbers 3
This feature has been added in Numbers 3.2, released 2014-04-01.  Numbers now honors the ⌘-{ and ⌘-} keystrokes for previous and next sheet, respectively.  Additionally, the menu items are now under the Window menu to be remapped.

Numbers 3.0 and 3.1 did not have this feature.