3,4 and 5 are are the XWindows button's that are triggered when you use the wheel. 4 and 5 are up and down (in some order). Anyway, it's the native X mouse buttons. It doesn't mean that API's represent it as such, as you have shown. In the C one it should, but I haven't written for the mouse wheel :)

Under the hood of Tk, it gets mouse button "signals" 3,4,5 and the internals of Tk will create events in turn that you receieve when you write event listeners.