How do make 3 dropdown menus dependant on each other.

HI GUYS,

I am new to livecycle designer and am building a form  that has 3 dropdown menus; 1st dropdown menu has a list of options e.g. E_COM, E_ENGS and by selecting this the second menu populates E_COM 1, E_COM 2 and when an item on the second menu is selected e.g E_COM 2, the third dropdown menus is populated with a list of options.

Please can anyone help? and show me how to write these in a Switch statement or how would you advice i do about doing this?.

thanks