  We have used archival infrared images obtained with the Wide Field Camera 3
on board the Hubble Space Telescope to constrain the initial mass function of
low-mass stars and brown dwarfs in the W3 star-forming region. The images cover
438 arcmin$^2$, which encompasses the entire complex, and were taken in the
filters F110W, F139M, and F160W. We have estimated extinctions for individual
sources in these data from their colors and have dereddened their photometry
accordingly. By comparing an area of the images that contains the richest
concentration of previously identified W3 members to an area that has few
members and is dominated by background stars, we have estimated the luminosity
function for members of W3 with masses of 0.03-0.4 $M_\odot$. That luminosity
function closely resembles data in typical nearby star-forming regions that
have much smaller stellar populations than W3 ($\lesssim$500 vs. several
thousand objects). Thus, we do not find evidence of significant variations in
the initial mass function of low-mass stars and brown dwarfs with star forming
conditions, which is consistent with recent studies of other distant massive
star-forming regions.
