The balls in 3 c are heavier and the cues "need" to be a bit stiffer. The shorter, conical taper etc provides that. They usually have a bigger joint (wider) also.
You can play with a poolcue also of course but when you get a bit more "advanced" you want some more "power" without loosing accuracy. They are looking for english even in the 5 rail etc.
using a 3 c cue in pool is doable also of course, down side that most feels is the conical taper and you will also notice when jacking up that the stiffness makes the cb jump more easily imo.
Hope I made sense

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Christian