如何将多个输入框的值传递给另一个单一输入或文本区域

var input1 = document.getElementById('input1').value;
var input2 = document.getElementById('input2').value;
var input3 = document.getElementById('input3').value;

var combinedValue = input1 + ' ' + input2 + ' ' + input3;

document.getElementById('targetInput').value = combinedValue;

<div id="app">
  <input v-model="input1" />
  <input v-model="input2" />
  <input v-model="input3" />
  
  <textarea>{{ combinedValue }}</textarea>
</div>

<script>
new Vue({
  el: '#app',
  data: {
    input1: '',
    input2: '',
    input3: '',
  },
  computed: {
    combinedValue() {
      return this.input1 + ' ' + this.input2 + ' ' + this.input3;
    },
  },
});
</script>

<form action="/submit" method="post">
  <input type="text" name="input1" />
  <input type="text" name="input2" />
  <input type="text" name="input3" />
  
  <input type="submit" value="Submit" />
</form>