Python script to update an existing column values in a csv from another two tables
Your code as it stands now will open and read file f2 every time you process a line from f1. This is way more work than you need to do.

I suggest you create a dictionary mapping ids to replacement values. You can then read all the lines from table2 one time, and populate the dictionary:

feed_desc = dict()

with open(table2) as tbl2:
    t2csv = csv.reader(tbl2)
    next(t2csv)  # skip headings

    for t2row in t2csv:
        feed_id = t2row[1]
        desc = t2row[2]

        feed_desc[feed_id] = desc


Once that is done, you can simply check if an id is in feed_desc, and if so provide the replacement value:

with ...
    ...
    for t1row in t1csv:
        feed_id = t1row[2]

        if feed_id in feed_desc:
            t1row[2] = feed_desc   # replace id with desc, you said.

        t1writer.writerow(t1row)


You can do a similar thing for t3 into a different dictionary. Just make sure you read both t2 and t3 before you start processing t1.