comparing column values when there are nothing in common between tables
like this perhaps.

WITH t1 AS ( SELECT col3 AS tbl1col3,row_number()OVER() FROM tbl1 limit 10)
    ,t2 as ( select col6 AS tbl2col6,row_number()OVER() FROM tbl2 limit 10)
    ,t3 as ( select col1 AS tbl3col1,row_number()OVER() FROM tbl3 limit 10)
SELECT tbl1col3, tbl2col6, tbl3col1 
FROM t1 
    FULL OUTER JOIN t2 USING (row_number)
    FULL OUTER JOIN t3 USING (row_number)
;