I am fabricating a three phase ac/dc converter.
Are you making DC from AC or what?

.so please tell me how to compare triangular wave with sinusoidal wave in software.in hardware we can do this by OPAMP.
Both are bad ideas, use a comparator (many '51 derivatives have one or more).
if u have any codes to realize PWM please give me.
get the PCA cookbook (address in links <---)

Erik