Three level menu

Hi!
I’m runing a site in WordPress 3.5.1 with three level menu like this:

Menu item 1
Menu item 2
Menu item 3
-Menu subitem 1
–Menu sub-subitem 1
–Menu sub-subitem 2
–Menu sub-subitem 3
-Menu subitem 2
-Menu subitem 3
-Menu subitem 4
Menu item 4
Menu item 5

I can choose between Menu items and Menu subitems and all the menu stays visible all the time. When I click on any Menu sub-subitem the whole menu shrinks to Menu items.
I have read many articles on this problematics and almost everyone describes how to add a line with a definition of depth of a menu. I’ve try to add the code in several php files and places but without any success. I’m using a custom made theme but I’m not the author and I can’t ask the author for help.
If anyone could give me some advice please.