They had three turrets and six barrels due to tonnage restrictions. Think of them as light heavy cruisers. That armament may indeed have been the minimum required for proper target saturation to ensure a reasonable chance of a hit, but the number of turrets and barrels was strictly governed by tonnage issues. They needed to stretch the available tonnage out and eliminating a turret here or there allowed for that. For the same reason, the Arethusas only had one after turret, as well; eliminate a turret and the associated weights enough times and you could just maybe afford another ship. That was the logic. Whether it resulted in more combat effective vessels is another matter entirely.

Bob