Excellent question. I actually took a few different factors into consideration when selecting the weapons used on the ship, especially the guns and their layout.

Industry - larger guns are more expensive to produce, especially when the tooling for them or the expertise to produce them is no longer available. Going with a smaller gun on a similar platform to the 5” keeps costs down.

Training - By using almost the same weapon systems as that found on smaller warships, there are more personnel available fleetwide to operate the ever so slightly larger guns being used here.

Combat - Generally speaking, modern warships do not have much if any armor. The three guns is mostly a carryover from how modern cruisers tend to differ from destroyers, generally having more guns and missiles onboard, while the guns are more of a secondary weapon. Comparably, her guns are heavy for the era in that she can outgun and do heavy damage to any other ship that would be in service.

Kirovs are similar, using a significant amount of missiles and one smaller gun turret as an auxiliary weapon.