If the input is
foo,bar,baz
bar,baz,qux
qux,quux,baz
bar,foo,qux
waldo,fred,garply
the output should be
foo,bar,baz
bar,baz,qux
waldo,fred,garply
As you can see, records are deduplicated based on the 3rd column's value. If multiple records have the same 3rd column value, pick a random one (or the first one; doesn't matter)
The idiomatic awk answer is awk -F, '!seen[$3]++' file
That will print a line the first time a value is seen in the 3rd column.
If you're not bothered about the order of the output, can just use sort, as below.
-t, sets the field delimiter to ","-k3 defines the sort key as the third field-u indicates that only unique results are wanted
$ sort -t, -k3 -u file
foo,bar,baz
waldo,fred,garply
bar,baz,qux
$
