I have media databases that I am not sure how to merge and am afraid of messing of the relationship structure:

Table 1 = list of tapes with portal to table 2

Table 2 = contents of tapes

Table 3 = table made from Final Cut Pro columns (we add to this one by importing batch lists made from Final Cut Pro). Each row may have the same tape, but different description of the content at that point in the tape.

Tables 1 and 2 = 1 to many relationship by unique id

How do I get one instance of a tape name from Table 3 into Table 1 and get all of the contents for each tape from Table 3 into Table 2?

The way this database is supposed to work is that for each tape you would be able to see (in the portal) the contents of that tape.

I've already achieved this with the way Tables 1 and 2 are now, but Table 3 doesn't have the unique ID like the others.

Not really answering your question, but usually raw data is imported into an intermediate table (or file) and then processed (often multiple times) to remove duplicates, generate unique keys, generate summary information and so on, which is then imported into the main tables.