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<input onChange="this.form.submit()" />
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<form id="formName" action="<?php echo $_SERVER['PHP_SELF'];?>" method="get">
<input type ="checkbox" name="cBox[]" value = "3" onchange="document.getElementById('formName').submit()">3</input>
<input type ="checkbox" name="cBox[]" value = "4" onchange="document.getElementById('formName').submit()">4</input>
<input type ="checkbox" name="cBox[]" value = "5" onchange="document.getElementById('formName').submit()">5</input>
<input type="submit" name="submit" value="Search" />
</form>