There are two ways to implement menus in 3D:

2D menu graphics + Z-depth

You can choose to implement your menu graphics as 2D images with a Z-depth value.  In this case, the graphics requirements are the same as they are for 2D.  The only difference is the addition of the Z-depth value, which determines how deep or how far in front of the screen the menus will be perceived by the viewer.  When you choose this method, your menus can be displayed by earlier (non-Profile 5) players (a firmware update will likely be required).

3D menu graphics

The BD3D Spec allows you to use 3D graphics for both HDMV and BD-J menus.  In this case, each graphic element of your menu can be displayed at a different depth.  Profile 5 increases the image buffer for 3D menus.  When you choose to implement full 3D menus, your menus can not be displayed by earlier players.  Only a Profile 5 player will display the menus.

Which approach you choose depends up the specific requirements of your disc as well as your available resources.  Backwards compatibility is another major factor to consider.