Why guaranteed availability can require four ships

Three ships are given as the usual minimum for one available, one in training or upgrading, and one in refit. Four provide more assurance by allowing for a further short refit or a ship returning from operations. The British and French four-boat ballistic-missile submarine forces illustrate that logic for keeping one boat continuously at sea. With only two ships, both can occasionally be operational, but sustained simultaneous availability is treated as rare. This is an availability rule of thumb built around maintenance and preparation cycles, rather than a count of hulls that can all deploy together.