  During 2022--2023 Z.-W. Sun posed many conjectures on infinite series with
summands involving generalized harmonic numbers. Motivated by this, we deduce
$31$ series identities involving harmonic numbers, three of which were
previously conjectured by the second author. For example, we obtain that \[
\sum_{k=1}^{\infty} \frac{(-1)^k}{k^2{2k \choose k}{3k \choose k}} \big(
\frac{7 k-2}{2 k-1} H_{k-1}^{(2)}-\frac{3}{4 k^2} \big)=\frac{\pi^4}{720}. \]
and \[ \sum_{k=1}^\infty \frac{1}{k^2 {2k \choose k}^2} \left(
\frac{30k-11}{k(2k-1)} (H_{2k-1}^{(3)} + 2 H_{k-1}^{(3)}) + \frac{27}{8k^4}
\right) = 4 \zeta(3)^2, \] where $H_n^{(m)}$ denotes $\sum_{0<j \le n}j^{-m}$.
