Why triple dual-purpose mounts bring more trouble than benefit

Is a triple dual-purpose naval gun mount practical?

It can be engineered, but the center gun complicates the mount and the extra bulk and weight usually do not justify the limited gain over twin mounts.

Triple 4-inch and 4.5-inch designs faced the familiar problem of a centre barrel interfering with its neighbours. More machinery and structure add cost without a proportionate increase in usable firepower. Twin mounts were the more practical option.