Calculating the complex signal's average power
Solve the integral:
$$
P = \lim_{T \to \infty} \frac{1}{T}\int_{-T/2}^{T/2} |x(t)|^2 dt
$$
This is usually unwieldy to solve directly; it can be shown that if a signal is periodic, you only need to integrate over the fundamental period $T_0$:
$$
P = \frac{1}{T_0}\int_{-T_0/2}^{T_0/2} |x(t)|^2 dt
$$

EDIT:

Another "trick" is to note that sinusoidal signals of different frequencies are incoherent, so when integrating over long periods of time, it is only necessary to find the power of the individual components because the cross terms go to zero. 

This means that
$$
P = P_1 + P_2 + P_3
$$
assuming $x_1$, $x_2$, and $x_3$ are incoherent.  Find the individual powers and sum them.