How could I have three or more input languages active (namely English/Russian/Chinese), but cycle with Alt+Shift only between two of them (English/Russian)?
Punto Switcher handles this task only with custom shortcuts like Caps Lock or right Ctrl.
On the Advanced Key Settings tab (in the Text Services and Input Languages dialog) you can change the keyboard shortcut for each language you have set.
By default, the "Between input languages" action is set to Left Alt+Shift, like you mentioned. The other actions ("To English", "To Russian", etc.) won't have a shortcut assigned until you manually set it to one of your liking.