Step 1: Use the energy formula for hydrogen atom.
The energy of the \(n^{\text{th}}\) orbit in hydrogen atom is given by
\[
E_n=\frac{-13.6}{n^2}\,\text{eV}
\]
Step 2: Find the energy of the ground state.
For the ground state,
\[
n=1
\]
Therefore,
\[
E_1=\frac{-13.6}{1^2}
\]
\[
E_1=-13.6\,\text{eV}
\]
Step 3: Find the energy of the third state.
The third state corresponds to
\[
n=3
\]
Thus,
\[
E_3=\frac{-13.6}{3^2}
\]
\[
E_3=\frac{-13.6}{9}
\]
\[
E_3=-1.51\,\text{eV}
\]
Step 4: Calculate the excitation energy.
Energy required to excite the electron from ground state to third state is
\[
\Delta E=E_3-E_1
\]
\[
\Delta E=(-1.51)-(-13.6)
\]
\[
\Delta E=12.09\,\text{eV}
\]
\[
\Delta E\approx 12.1\,\text{eV}
\]
Step 5: Final conclusion.
Therefore, the required excitation energy is
\[
\boxed{12.1\,\text{eV}}
\]