If you use the form from www.uscis.gov. You need to follow the instruction.
2。一家三口同时申请AP,申请费是否可以写在一张支票上(总额=170X3)?/还是分开写三张更好些?
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.