只要有第三位就清除：

<input type="text" onkeyup="value=value.replace(/\.\d{2,}$/,value.substr(value.indexOf('.'),3))" />

	     	 jQuery.validator.addMethod("lrunlv",function(value,element) {       
			     return this.optional(element) || /^\d+(\.\d{1,2})?$/.test(value);       
			 },"小数位不能超过三位");   

 //验证小数点后的位数
 jQuery.validator.addMethod("decimals",element,d){
 	 var a = value.indexOf(".")+1;
 	 if(a==0){
 	 	a=value.length;
 	 }
 	 var b = value.length;
 	 var c = b-a;
     return this.optional(element) || c<=d; 
 });