filter by three state button in header

Hello,

I’ve created a grid with 3 columns. (ed,ch,ch).
Above the grid is a header with #combo_filter,{#clear_button},{#clear_button}
For each clear_button I create a ‘three-state’-button-checkbox (On/Off/Disable)

mygrid._in_header_clear_button=function(tag,index,data){
	tag.innerHTML="<input type='image' src='../dhtmlx/imgs/ChOffBwDis.png'  />";
	var grid = this;
	tag.firstChild.onclick=function(){
	if (aanuitknop[index]==1){
		aanuitknop[index]=0;
		this.setAttribute("src", '../dhtmlx/imgs/ChOffBw.png')
		mygrid.filterBy(index,'0');
	}
	else if (aanuitknop[index]==2){
		aanuitknop[index]=1;
		this.setAttribute("src", '../dhtmlx/imgs/ChOnBw.png')
		mygrid.filterBy(index,'1');
	}
	else { 
		aanuitknop[index]=2;
		this.setAttribute("src", '../dhtmlx/imgs/ChOffBwDis.png')
		mygrid.filterBy(index,0);
		}
	}
};

If I click on the button the image of the button changes and the grid is filtering on the checkbox.
However, if the filter in column 2 is checked, the filter in column 3 or column 1 is forgotten.

Any Idea to solve this.

Regards,

Laurens