No easy way to do that given three separate join tables. One option would be to use a merged table, perhaps with three different link field for the three different occurrences of your employee table or perhaps just a "type" or "category" field in the Join table so that your portals can use filters to exclude all but one category of records...
Wow, sounds complicated. Maybe the merge table is the easiest, what do you think? Never made a merge table can you explain the ins and out?
Not a merge table, but rather "merge your records into a single join table"--which can be done with import records.