How are these road signs installed before a railway crossing?

1. 1Independently, at an equal distance between the signs "Railway Crossing with Barrier" or "Railway Crossing without Barrier".
2. 2Together with the first of the signs "Railway Crossing with Barrier" or "Railway Crossing without a Barrier."
3. 3Together with the second of the signs "Railway Crossing with Barrier" or "Railway Crossing without a Barrier."