  By using images from the HST/WFPC2/ACS archive, we have analyzed the spatial
distribution of the AGB and RGB stars along the galactocentric radius of nearby
spiral galaxies M~81, NGC 300 and NGC 55. Using color-magnitude diagrams and
stellar luminosity functions, we gauge the stellar contents of the surroundings
of three galaxies. The red giant population (RGB) identified at large
galactocentric radii yields a distance of $3.85\pm0.08$ Mpc for M~81,
$2.12\pm0.10$ Mpc for NGC 55, and $2.00\pm0.13$ Mpc for NGC 300, and a mean
stellar metallicity of $-$0.65, $-$1.25, and $-$0.87. We find that there are
two number density gradients of RGB stars along the radius, which correspond to
the thick disk and halo components of the galaxies. We confirm the presence of
metallicity gradient of evolved stars at these galaxies, based on the
systematic changes of the color distribution of red giant stars. These results
imply that thick disk might be a general feature of the spiral galaxies, and
endorse a further investigation of the outer stellar edges of nearby spirals,
which is critical in constraining the origin and evolution of galaxies.
