Why shrinking a Town-class cruiser created a poor compromise

Should the Crown Colony-class light cruisers have been designed with three turrets from the outset instead of trying to fit Town-class capability onto about 1,500 fewer tons?

Yes. Starting with a smaller armament would have been more coherent than trying to compress an already balanced Town-class concept into a lower displacement.

Beginning with three main turrets would have been more coherent than compressing a Town-class concept into a substantially smaller displacement. The historical effort sought a near-full-length Town with less weight, treating armour as something that could be fitted around the remaining design. That approach preserved political ambitions but made the naval architects reconcile capabilities that no longer fit comfortably together. A smaller warship is not automatically a cheaper version of a balanced larger design because removing displacement changes the whole design problem.