How to set the camera's clearflags

pls help me. now I encounter a problem. there are two camera in my scene, the gui camera is used to the UI element, and the main camera is used to others. the gui camera's clearFlags is set to depth only ,and the main camera's clearflags is skybox,and the gui camera's depth is 1,and the main camera's depth is 1, now I want to add a new camera into the scene which is used to a gameobject,and the new camera's depth is 2,and its clearflags is depthonly,but I can only see the new camera's view and the main camera'view,can't see the gui camera's view,and the game view is flickering. any one experience this before ? pls help me.