If you join-keeping-all from file1 to file2 making file2, file 2x to file3 getting file 3x, you end up with a variable length line like:

key,#,#,#

I have a many to one join with keep-all I wrote that allows pipes (no seeks, file 1 has to be the many and file 2 the one), m1join.c, because I love pipes and hate temp files and step by step.

From there, it is not hard to process it, like with sed into ksh input:

echo "key<tab>$(( # + # + # ))"