Form Report Button

[NLA]

i am having great difficulty with conditional expression/syntax/sequence (if, else if) in Access 2007.

I have a form w/ 3 combo boxes and a single report button that i would like to do the following:
1- Display msgbox if all combo boxes are null OR if more than one combo box has selected value THEN
2- clear combo boxes on form (for user to re-select)
3- run report for the one selected combo box value

[June7]

Post attempted code for analysis.