I'd like to create a DTS package to update records within a table
leaving other information stored in those records as is. For example,
table a has a record with 5 columns where 3 are null. An external
source has the non-null values for those 3 columns. Can anyone help
with some general steps as to how I might fill those 3 columns in the
sql table?

If the source is the text file. u must first import text source into
table.
then exec update statement in sql query task.