As far as I know most 1 piece cranks are not replaceable with 3 piece cranksets and most bikes with 1 piece cranks, unless vintage, aren't worth the price of a good 3 piece crankset. As far as measuring it I don't really know what kind of measurements you're taking about. Crank length is measured from the center of the bottom bracket to the center of the pedal hole. Bottom brackets are sized by their threading, most often English, and their spindle length, which is governed by your choice of cranks. Of course this is all pointless if you can't put a 3 piece crank on your bike. I would recommend taking your bike in to a bike shop and having them figure it out.