Do I have to use all 3 of the ports on the relay shield made by seeed studios

Im making a rov and I would like to know if I need to use all 3 of the ports on the relay shield when I want to control motors? Also does anybody know what I should use for the camera. Also I was thinking not have any electronics in under water beside the actual motors and camera. The lights are going to have a different power source so they dont drain the battery for the motors and camera.