Ok, so I stumbled upon the following explanation of how the Barbarian's "sweep attack" works. To make a long story short: the to-hit % for the right and left squares of a sweep attack is way lower than for the main (center) square you're attacking. At low levels, the Barbarian has low dex and low to-hit % to begin with, and lowering it even further means I'm simply not hitting the left and right monsters. Once I get a massive +% to hit weapon, I expect to start hitting all 3 monsters most of the time. (side note: the damage done to the side monsters is apparently only 1/4 of the main damage).
Full explanation from http://www.oocities.org/daslonghorn/mmasguide.html:
"Quarter damage, also referred to as sweep attack...see below for a
small diagram. It is basically a penalty, because:
1) The ToHit% you get from the formula further down is subtracted from
your actual character ToHit%, this ToHit% is then applied to the
targets which are affected by the sweep attack 2) Damage for sweep
attack is 1/4th of your normal damage. However, to me it seems this
The quarter damage ToHit penalty is 70 - 2*clvl, with a minimum
ToHit%-penalty of 30% (so it is "neutralized" at clvl 20). That's
still not much, and you will find you hit all three guys in most cases
(unless there are only two). That check is applied to the two squares
to the left and to the right of the square you attack (left and right
from your Barbarian's point of view)."