  The $\gamma$-ray emitting compact symmetric objects (CSOs) PKS 1718--649, NGC
3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order to
investigate their $\gamma$-ray radiation properties, we analyze the $\sim$14-yr
Fermi/LAT observation data of the three CSOs. They all show the low luminosity
($10^{41}-10^{43}$ erg s$^{-1}$) and no significant variability in the
$\gamma$-ray band. Their $\gamma$-ray average spectra can be well fitted by a
power-law function. These properties of $\gamma$-rays are clearly different
from the $\gamma$-ray emitting CSOs CTD 135 and PKS 1413+135, for which the
$\gamma$-rays are produced by a restarted aligned jet. In the
$L_{\gamma}-\Gamma_{\gamma}$ plane, the three CSOs are also located at the
region occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 display
the similar feature to blazars. Together with the similar radio emission
property to $\gamma$-ray emitting RGs Cen A and Fornax A, we speculate that the
$\gamma$-rays of the three CSOs stem from their extended mini-lobes. The
broadband spectral energy distributions of the three CSOs can be well explained
by the two-zone leptonic model, where their $\gamma$-rays are produced by the
inverse Compton process of the relativistic electrons in extended region. By
extrapolating the observed Fermi/LAT spectra to the very high energy band, we
find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov
Telescope Array in future.

High Energy Astrophysical Phenomena