To answer your first question, in phpMyAdmin, open tab2, Operations > Copy table, choose tab1, and select Data only.
If you really want a tab3, open tab1, Operations > Copy table (structure and data) to tab3, then open tab2, Operations > Copy table (data only) to tab3.
in phpMyAdmin there is no option in the Operations dialog to ignore a duplicate entry. However you could do it via a SQL command like suggested in the answer below, but using the IGNORE keyword. See mysqltutorial.org/mysql-insert-ignore. –