join in triggers

i need help in creating trigger where one can retrieve data from two or more tables and insert the data into a new table. can anyone plz suggest a solution for this?

The query can be ....
insert into table3 (col1,col2)
select col1.table1 as col1,col1.table2 as col2 from table1,table2 where table1.common_field = table2.common_field


But at which level u want the trigger.If the above query is ur solution then its ok otherwise please elaborate abt ur requirement...

Jignesh