The bit pattern returned represents the columns that were modified. You AND the returned value with a mask that represents the column of interest.

COLUMNS_UDPATED() & 1 = 1 => first column changed
COLUMNS_UPDATED() & 2 = 2 => second column changed
COLUMNS_UPDATED() & 4 = 4 => third column changed
COLUMNS_UPDATED() & 7 > 0 => one or more of columns 1,2, or 3 have changed.

-Mike