How to compute the energy of two signals?
You simply have to apply the definition of energy. Assuming all signals involved are real, the energy of $g_3(t)$ is given by

\begin{align}
E &amp;= \int_{-\infty}^\infty g_3^2(t) \, dt \\
&amp;= \int_{-\infty}^\infty \big(g_1(t) + g_2(t)\big)^2 \, dt \\
&amp;= \int_{-\infty}^\infty g_1^2(t) \, dt + 2\int_{-\infty}^\infty g_1(t)g_2(t) \, dt + \int_{-\infty}^\infty g_2^2(t) \, dt.
\end{align}

One interesting case is where $g_1(t)$ and $g_2(t)$ are orthogonal, in which case the middle integral is zero. In that case, the energy of $g_3(t)$ is the energy of $g_1(t)$ plus the energy of $g_2(t)$. One simple example is two rectangular pulses that do not overlap.