find the H C F of three Numbers 198 126 and 240 by using fundamental theorem of arithmetic

Use one of the methods described here:

http://easycalculation.com/what-is-hcf.php
198 = 99*2 = 2*3^2*11
126 = 2*3^2*7
240 = 15*16 = 2^4*3*5

The HCF is 2*3 = 6