To provide navigational certainty the vessel’s navigator visually aligns one set of three up with the other (i.e. they act as a pair) and because each of the three lights is a different colour the navigator can be certain he’s lining up the correct lights.
Comments
A relatively simple system of lights is used at Portsmouth Harbour to help the aircraft carriers navigate the narrow channel in and out of port.
This looks to be similar to the 3-light system that the author talks about.
https://insidedio.blog.gov.uk/2016/08/22/safe-approach-to-po...