  The stellar contents of the open clusters King 12, NGC 7788, and NGC 7790 are
investigated using MegaCam images. Comparisons with isochrones yield an age <
20 Myr for King 12, 20--40 Myr for NGC 7788, and 60 -- 80 Myr for NGC 7790
based on the properties of stars near the main sequence turn-off (MSTO) in each
cluster. The reddening of NGC 7788 is much larger than previously estimated.
The luminosity functions (LFs) of King 12 and NGC 7788 show breaks that are
attributed to the onset of pre-main sequence (PMS) objects, and comparisons
with models of PMS evolution yield ages that are consistent with those measured
from stars near the MSTO. In contrast, the r' LF of main sequence stars in NGC
7790 is matched to r' = 20 by a model that is based on the solar neighborhood
mass function. The structural properties of all three clusters are investigated
by examining the two-point angular correlation function of blue main sequence
stars. King 12 and NGC 7788 are each surrounded by a stellar halo that extends
out to 5 arcmin (~ 3.4 parsecs) radius. It is suggested that these halos form
in response to large-scale mass ejection early in the evolution of the
clusters, as predicted by models. In contrast, blue main sequence stars in NGC
7790 are traced out to a radius of ~ 7.5 arcmin, with no evidence of a halo. It
is suggested that all three clusters may have originated in the same
star-forming complex, but not in the same giant molecular cloud.
