How to validate checkbox group with Vuetify

Why don't you define your selected field of your model an array, and bind it the checkboxes like below?

<v-checkbox class="pr-6" v-model="selected[0]" label="A" value="a"></v-checkbox>
<v-checkbox class="pr-6" v-model="selected[1]" label="B" value="b"></v-checkbox>
<v-checkbox class="pr-6" v-model="selected[2]" label="C" value="c"></v-checkbox>

For validation:

return this.selected.filter(s => s === true).length > 0

It can be done with an array as v-model. Just use computed property for validation:

computed: {
    rules() {
      return [
        this.selected.length > 0 || "At least one item should be selected"
      ];
    }
  }

Here is the Codepen

Also pay attention how hide-details property of v-checkbox is used.