<div class="layui-form-item">
  <label class="layui-form-label">xxx</label>
  <div class="layui-input-block">
    <input class="education" lay-filter="hope" type="checkbox" name="param[hope_employ][]" value="yy" title="xx">
  </div>
</div>

form.on('checkbox(hope)',function(data){
  console.debug(data);
  var len=$(".education:checked").length;
  if(len>3){
    $(data.elem).next().attr("class","layui-unselect layui-form-checkbox");
    $(data.elem).prop("checked",false);
    layer.msg('最多只能选3项！',{icon:5});
    return false;
  }
});