As you're wandering around Medici, you may notice encounters marked by a white circle. These are random encounters. There are several types of encounters; these can include grapple challenges (such as opening a gate or getting a boat in the water), rescue missions, assassinations, vehicle deliveries, etc. These can unlock supply points or monasteries. It seems to be random as to which they unlock but in order to unlock this achievement, you must complete three encounters that unlock the monasteries. There is another achievement related to random encounters which is unlocked in the same way so I would recommend going for both at the same time