Hot water only works when additional showers are on as well.

[Researchjay]

While in my master shower the water is hot. However, you turn on a second shower on and my master shower gets hotter and the second shower stays cold. When I turn on a third shower the second shower now gets hot, the master shower stays hot and the third shower stays cold.

Why does my master shower get hotter when I turn on the second shower? And why does the second shower only get hot if I turn on the third shower?