Parent.vue

<template>
  <Child1 v-model:firstDropdownValue="firstDropdownValue" 
          v-model:secondDropdownValue="secondDropdownValue"/>
  <Child2 />
  <Child3 />
</template>

<script>
export default {
  data() {
    return {
      firstDropdownValue: '',
      secondDropdownValue: '',
      child2Data: {},
      child3Data: []
    }
  },
  methods: {
    submit() {
      // use all child data here
    }
  }
}
</script>

<style scoped>

</style>