The way the NieR team is used depends on how much you've invested into each character.
Full Team Rotation
Either at high investment (all characters at SSS-Rank or above with 6★ weapons) or at low investment (all characters at S-Rank with 5★ weapons), this composition can be used. In this setup, each character will take turns swapping onto the field.
This is the composition you'll usually use when A2 is at SSS-Rank with her 6★ weapon and both 2B and 9S are at S-Rank with 5★ weapons. A2 will be on the field at all times.