Calculating magnitude of a transfer function
First off, we transform the s domain to frequency domain with \$s=j\omega \$
which gives us:

$$T(j\omega) = \frac{1}{(j\omega+1)((j\omega)^2+j\omega+1)} = \frac{1}{(j\omega+1)(j\omega+1-\omega^2)} = \frac{1}{1-2\omega^2+j(-\omega^3+2\omega)}$$

Notice i wrote the denominator in the form \$a+bj\$.
Taking the magnitude of this function gives:

$$\left |T(j\omega)\right |=  \frac{1}{\sqrt{a^2+b^2}} = \frac{1}{\sqrt{(1-2\omega^2)^2+(-\omega^3+2\omega)^2}} = \frac{1}{\sqrt{(4\omega^4-4\omega^2+1)+(\omega^6-4\omega^4+4\omega^2)}} =\frac{1}{\sqrt{1+\omega^6}}  $$