It takes a lot of manpower to operate a navy and manpower equates to salaries ! Every capital ship should have a minimum of three crews with four being the most desirable. Most navies around the world can barely muster enough sailors to man their ships with two crews and most of time their ships are in dock due to a lack of manpower.
My understanding is that most naval ships in the world only have a single crew.
Although there are some exceptions to this that I'm aware of.
The LCS which previously had 2 crews comes to mind, although they've changed back to a single crew now.
Or the Ohio SSBNs which have 2 crews, which maximises the time at sea and readiness.
So I'm curious as to where the idea of 3-4 crews for a ship comes from.