Working with triggers

Hello friends!
Let's consider, we have 3 tables:

tabel1:
-------
id | subjects | local_idx
-------------------------
1 | s1 | 1
2 | s2 | 25
3 | s3 | 14
...


A group has got some subjects only one time, e.g group a1 has subject s1 only one time:

table2:
-------
id | groups | local_idx | subjects
---------------------------------------------------
1 | a1 | 1 | s1
2 | a2 | 2 | s1
3 | a1 | 1 | s2
5 | a2 | 2 | s3
6 | a3 | 3 | s1
7 | a1 | 1 | s3
8 | a2 | 2 | s2
...


Everytime we insert new record in table2, table3 must have such kind of results:

table3:
-------
id | local_idx (from table2) | local_idx (from table1)
------------------------------------------------------
1 | 1 | 1
2 | 2 | 1
3 | 1 | 25
4 | 2 | 14
5 | 3 | 1
6 | 1 | 14
7 | 2 | 25
...

For example, when we insert (in table2) group a3 with local_idx 3 and subject s2 (which local_idx's 14)
then result must be:
8 | 3 | 14

 

Any ideas, please.

 

May be triggers can't help me... :-(

 

Triggers are used to give the ristrictions in performing applications example:
A bank wants to stop the transaction on sunday , So this can be done in database using triggers.

 

Thank You.