<div id="warp">
 <ul id="menu">
 <li class="firstLevel" v-for="(firstList,index) in menuData">{{firstList.title}} 
  <ul v-if="firstList.value">
  <li class="secondLevel" v-for="(secondList,index) in firstList.value">{{secondList.title}}
   <ul class="thirdLevel">
   <li v-for="(thirdList,index) in secondList.value2">{{thirdList}}</li>
   </ul>
  </li>  
  </ul>
 </li>
 </ul>
</div>

var app = new Vue({
 el:"#warp",
 data:{
 menuData: {
     firstList:{
     title:'練習(xí)冊1', 
     value: [
      {
      title:"物理",
      value2:[
       "暑期",
       "寒假",
       "周末"
      ]
      },
      {
      title:"化學(xué)",
      value2:[
      "暑期",
      "寒假",
      "周末"
      ]
      },
     ],
   },
   secondList:{
    title:'練習(xí)冊2', 
     value: [
      {
      title:"政治",
      value2:[
       "暑期",
       "寒假",
       "周末"
      ]
    },
      {
      title:"地理",
      value2:[
      "暑期",
      "寒假",
      "周末"
      ]
      },
     ],
   },
   thirdList:{
    title:'練習(xí)冊3', 
     value: [
      {
      title:"語文",
      value2:[
       "暑期",
       "寒假",
       "周末"
      ]
    },
      {
      title:"數(shù)學(xué)",
      value2:[
       "暑期",
       "寒假",
       "周末"
      ]
      },
      {
      title:"英語",
      value2:[
       "暑期",
       "寒假",
       "周末"
      ]
      },
     ],
   }
 },     
 },
});

body{
 max-width: 640px;
 margin: 0px auto;
 font-size: 14px;
 color: #666666;
 background-color: #ffffff;
}
ul{
 padding: 0px;
}
li{
 list-style: none;
}
#menu{
 display: flex;
 justify-content: space-between;
}
.firstLevel{
 position: relative;
 min-width: 100px;
 cursor: pointer;
}
.secondLevel{
 display: none;
 margin-left: 10px;
}
.firstLevel:hover .secondLevel{
 display: block;
}
.thirdLevel{
 display: none;
 margin-left: 15px;
 width: 40px;
}
.secondLevel:hover .thirdLevel{
 display: block;
}