父组件

<template><div><h1>我是父组件 {{num}}</h1><hr><child></child></div>
</template><script setup>
import child from './child.vue';
import { ref,provide } from 'vue';
let num = ref(520)
provide('parentNum',num)
</script>

子组件

<template><div><h2>我是子组件</h2><hr><grandchild></grandchild></div>
</template><script setup>
import grandchild from './grandchild.vue';
</script>

孙子组件

<template><div><h3>我是孙子组件</h3><p>这是爷爷的值 {{parentNum}}</p><button @click="handler">点击我爷爷的值减100</button></div>
</template><script setup>
import { inject } from 'vue';
let parentNum = inject('parentNum')
const handler = () => {parentNum.value -= 100
}
</script>