Because of the way that pow and sqrt are written internally, it is usually much more efficient to multiply doubles rather than using these functions for squaring operations.

{
Sum_of_sq1 = Side1*Side1 + Side2*Side2;
Sum_of_sq2 = Side1*Side1 + Side3*Side3;
Sum_of_sq3 = Side2*Side2 + Side3*Side3;
if (Sum_of_sq1==Side3*Side3 || Sum_of_sq2==Side2*Side2 || Sum_of_sq3==Side1*Side1)
printf("The triangle is a right triangle.\n");
}