[рони]

Идентификаторы должны быть уникальными на странице.

shtopor,

<script type="text/javascript">
	window.onload = function(){
	    var count = 0;
        document.getElementById("check0").onclick = function(){
	        document.getElementById("input").value = (this.checked) ? count+=150 : count-=150;
	    };
	    document.getElementById("check1").onclick = function(){
	        document.getElementById("input").value = (this.checked) ? count+=150 : count-=150;
	    };
      document.getElementById("check2").onclick = function(){
	        document.getElementById("input").value = (this.checked) ? count+=150 : count-=150;
	    };
	};
	</script>

	<input type="checkbox" id="check0" value=""  />
    <input type="checkbox" id="check1" value=""  />
    <input type="checkbox" id="check2" value=""  />
	<input type="text" id="input" value="0" />