  We work out a specific class of three-loop diagrams (of order $\alpha_s^2$)
contributing to the decay amplitude for $b \to s \gamma$ associated with the
current-current operators $O_1$ and $O_2$ at the physical value of the
charm-quark mass $m_c$. For many of the considered diagrams we were able to
solve the master integrals using differential equations in the canonical form.
For some diagrams we did not find a transformation to canonical form and
therefore calculated the corresponding master integral directly as an expansion
around $z=m_c^2/m_b^2=0$, retaining power terms up to $z^{10}$ and keeping the
accompanying $\log(z)$ terms to all powers. The results for the sum of all
considered diagrams are given in tabular form, while contributions of
individual diagrams (or combinations thereof) are given in electronic form.
