The three cities

"The Three Cities is a collective description of the three fortified cities of Birgu, Senglea and Cospicua in Malta. The oldest of the Three Cities is Birgu, which has existed since the Middle Ages. The other two cities, Senglea and Cospicua, were both founded by the Order of Saint John in the 16th and 17th centuries." (Source)

As per recommendation from our host, we only stopped in Birgu as we had a good bus connection from Marsascala. 

"Birgu is a very old locality with its origins reaching back to medieval times. Prior to the establishment of Valletta as capital and main city of Malta, military powers that wanted to rule the Maltese islands would need to obtain control of Birgu due to its significant position in the Grand Harbour. In fact, it served as the base of the Order of Saint John and de facto capital city of Malta from 1530 to 1571. Birgu is well known for its vital role in the Great Siege of Malta of 1565." (Source)

As we were there on a very very very hot day, not that many people were on the streets, which was quite cool as you had the city to yourself 

the streets are so incredibly cute, I could add 100 pics, but I will limit myself as otherwise it will be nothing for you to discover ;) 

As I was saying it was a super hot day so, we every 1-2h had to stop for a break, and it was a delight to try the local cheese (Gozo cheese)

Crossing over to Valletta is by ferry which is great as you can just easily get an amazing 360 view. 

of course the boat above was not the ferry unfortunately ;).
To visit the whole city in detail you won't need more than half a day, so I say you can easily plan a full day for all the 3 cities and you might even get enough time to have dinner in Valletta itself ;)