How to optimize this Postgresql query

Question

Table A has columns (P1, P2, Value1) where P1 and P2 are a multi column primary key. Table B has columns (P3, P1, Q1) where P3 is the primary key. Table C has columns (P2, Q1, Value2) where P2 and Q1 are a multi column primary key.

I need to delete all rows from table A which don't have corresponding data in table B and C meaning to delete (p1,p2,v1) from A if there is no (p3,p1,q1) in B and (p2, q1, v2) in B for any q1 and v2.

Currently the query is:

DELETE FROM A where concat(P1, P2) NOT IN
(
SELECT Distinct On (A.P1, A.P2)
        concat(A.P1, A.P2)
        from C
        INNER JOIN B
        ON C.Q1 = B.Q1
        INNER JOIN A
        ON A.P1 = B.P1
        WHERE A.P2 = C.P2
)

but this query never finishes on the tables where table A and C have millions of rows. How can I optimize the query?

Edit: I am trying to achieve what I wrote with the purpose of cleaning up table A periodically when its data becomes irrelevant which is determined by whether the data appears in tables B and C or not.

Edit2: Improved version:

DELETE FROM A where NOT EXISTS
    (
    SELECT 1
            from C
            INNER JOIN B
            ON C.Q1 = B.Q1
            AND A.P1 = B.P1
            AND A.P2 = C.P2
    )

Three questions:

1. Is using AND inside ON like that faster than using WHERE?
2. Can "with" be utilized to improve this further?
3. Any other suggestions of optimization?