hide combobox if it's value is null and show another

I have combobox1 , combobox2 ,combobox3 ,combobox4 ,combobox5 ,combobox6
and button1
the comboboxes are getting their data from sql tables
combobox4 ,combobox5 ,combobox6 are hidden by default

combobox1 , combobox2 ,combobox3 disabled by default

I want combobox1 ,combobox2 ,combobox3  to hide themselves if their value is null and

 show the other hidden ones which are combobox4 ,combobox5 ,combobox6 and if

combobox1 ,combobox2 ,combobox3 have value stay disabled and keep

combobox4 ,combobox5 ,combobox6 hidden


and the event of button1 check which 3 comboboxes are hidden and use the data in the visible ones to insert them into the table


thanks in advance