Need procedure help to insert entries to new table from two tables

[PK S]

HI team,
I am new to procedures i have a requirement and not sure how to start to achieve it.
I have 3 tables
Table_A, Table_B, Table_C

need to insert entries to Table_A by joining Tble_B and Table_C on bellow conditions
1. all records from table_C which are not in table_B
2. all records form table_B
3. If same records already present in table_A then dont insert same records from Table_B or Table_C
I need to write a stored procedure in oracle for this requirement. can some one help on this plz

Thanks
sk