Testing one application that's dependent on others


I am testing a mobile application that is dependent on two other applications. I am wondering what is the best practice to implement as switching between applications will need more time to execute. I am trying to call the back-end APIs to simulate the other applications’ actions. So this is an open discussion on what everyone thinks the implementation should be as I am trying to test one application only instead of the 3 applications at the same time.

Thank you