The record A:B:C = 10:12:15 means, by definition, that there is a quantity x such that A = 10x, B = 12x, C = 15x, and this quantity "x" is called "a common measure". So, in our case, that means that the common measure is 220 Hz = 10x; hence, x == 22 Hz. Then other two notes are 12*22 = 264 Hz and 15*22 = 330 Hz.