  The electronic structures of UX$_3$ (X=Al, Ga, and In) were studied by
photoelectron spectroscopy to understand the relationship between their
electronic structures and magnetic properties. The band structures and Fermi
surfaces of UAl$_3$ and UGa$_3$ were revealed experimentally by angle-resolved
photoelectron spectroscopy (ARPES), and they were compared with the result of
band-structure calculations. The topologies of the Fermi surfaces and the band
structures of UAl$_3$ and UGa$_3$ were explained reasonably well by the
calculation, although bands near the Fermi level ($E_\mathrm{F}$) were
renormalized owing to the finite electron correlation effect. The topologies of
the Fermi surfaces of UAl$_3$ and UGa$_3$ are very similar to each other,
except for some minor differences. Such minor differences in their Fermi
surface or electron correlation effect might take an essential role in their
different magnetic properties. No significant changes were observed between the
ARPES spectra of UGa$_3$ in the paramagnetic and antiferromagnetic phases,
suggesting that UGa$_3$ is an itinerant weak antiferromagnet. The effect of
chemical pressure on the electronic structures of UX$_3$ compounds was also
studied by utilizing the smaller lattice constants of UAl$_3$ and UGa$_3$ than
that of UIn$_3$. The valence band spectrum of UIn$_3$ is accompanied by a
satellite-like structure on the high-binding-energy side. The core-level
spectrum of UIn$_3$ is also qualitatively different from those of UAl$_3$ and
UGa$_3$. These findings suggest that the U~$5f$ states in UIn$_3$ are more
localized than those in UAl$_3$ and UGa$_3$.
