One of the possible solutions I think.
function resize(sender, width, height, oldWidth, oldHeight, eOpts)
{
Ext.defer(function() {
var me=sender.items,
dV=0;
//first panel 50%
dV+=50*width/100;
me.getAt(0).el.setStyle("width", dV+"px");
//second panel 30%
me.getAt(1).el.setStyle("left", dV+2+"px");
dV+=30*width/100;
me.getAt(1).el.setStyle("width", dV+"px");
//last 20%
me.getAt(2).el.setStyle("left", dV+2+"px");
dV+=20*width/100;
me.getAt(2).el.setStyle("width", dV+"px");
}, 20);
}