“Three Cities” can sound like three separate ferry stops. For this service, the landing is Cospicua/Bormla, from where you continue on foot toward the other parts of the Three Cities.
One harbour, two route boards
Valletta Ferry Services publishes a Sliema route and a Valletta–Three Cities route. Do not read the first timetable you see and assume it covers both. The departure columns for Cospicua and Valletta are separate, and winter, summer, Sundays and public holidays can change the useful return window.
Use the Valletta–Three Cities route page to frame the journey, then check the operator’s current timetable before leaving. If your plan involves Sliema too, see how to plan that return separately.