同じv-rowの中に入れ込む
<v-form v-model="valid" v-col>
<v-container>
<v-row>
<v-col cols="12" md="3">
<v-text-field v-model="firstname" :rules="nameRules" :counter="10" label="施主名" required></v-text-field>
</v-col>
<v-col cols="12" md="3">
<v-text-field v-model="lastname" :rules="nameRules" :counter="10" label="戸主名" required></v-text-field>
</v-col>
<v-col cols="12" md="3">
<v-text-field v-model="email" :rules="emailRules" label="住所" required></v-text-field>
</v-col>
<v-btn cols="12" md="3" class="ma-2" :loading="loading" :disabled="loading" color="secondary" @click="loader = 'loading'">
Accept Terms
</v-btn>
</v-row>
</v-container>
</v-form>