i want to write trigger for 3 tables and any insert to these 3 tables should update in the 4th table(combination of 3 tables based on join)

i want to write trigger for 3 tables and any insert to these 3 tables
should update in the 4th table(combination of 3 tables based on join)

i have 3 tables A,B,C. I have a stored procedure of these 3 tables with
join condition inserted in 4th table D. now what i want is "ANY INSERT OR
UPDATE"made to these 3 tables(A,B,C) should update 4th table D. Can any
one please tel how to write trigger in pl/sql?