null value

Hi,
I have a boolean in my webpage. 
to mimic the 3state checkbox i created a user control with the 3 images 
if image1 click so  intValue =1;  
if image2 click so  intValue=2;
if image3 click so intValue=3;
 
if the intValue is 3, i want to make the boolean value null, 
how I can do that. 
I thought I could set the value to null. Because I know if the dropdownlist control was never selected so the value passes as null, and for checkbox passes as null as well if it was never toogle. but for some reason I am getting error when   BooleanValue= intValue //intValue =null;
how i solve this I should be able to set the value to null.  

 
2) maybe someone can refer me to some good example of triState checkbox. 

 
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