How early triple turrets could reshape HMS Dreadnought

Would a semi-dreadnought with triple turrets but older machinery change the naval arms race?

Probably little by itself; widespread adoption of triple turrets during the pre-dreadnought era could change later battleship designs.

A single ship with triple turrets and older machinery would probably have little effect on the arms race. A wider change would follow if navies adopted triple main-gun mountings during the pre-dreadnought period, when such arrangements were already being considered. In this reconstruction, triples replace Dreadnought's wing turrets with a superfiring centerline arrangement, producing a longer, narrower ship with fifteen guns and perhaps 24 knots. Earlier American adoption could encourage other navies to follow. The identification of Kearsarge as a class considered for triples remains a tentative recollection.