As you have no link between the two tables data, you should have to use one cursor to create an array with first table and another to run in the second table, and, for each line, get the next value from the array and insert into the third table.
Other way could be (I dont know) stablish the first column in the third table as unique, and building an unique index for the second and third column, this way, you could make an insert like this.
"insert into table_c (col1, col2, col3)
select table_A.col1, table_B.col2, table_B.col3
from table_A, table_C"
If your DB Server inserts everything correct and just warns about errors, will insert the correct lines and warn about all the others.
Sorry but I haven't tested it, this is just theory, a clue. Anyway I hope it helps you
