If you use the form from www.uscis.gov. You need to follow the instruction.
2。一家三口同時申請AP,申請費是否可以寫在一張支票上(總額=170X3)?/還是分開寫三張更好些?
It does't matter. Law firm only use one check. For more convince for the uscis officer, separting them into three check is better because you file three forms.