open the master file.

open a temp file.

read a record, skipping header records, until a significant record or end of file.

Split the significant record (see the split function) into fields.  Throw out all but the last three fields and the key column.

loop through the pointers to the child files
  open each child file
  read the child file, checking for the key value.
  write the appropriate data to a temp file.
  close the child file.

repeat until all records in master file have been processed.

close the master file.
close the temp file
open it for appending

open the temp file for reading

read through the temp file, copying records to the end of the master file.