Uniendo 3 tablas Oracle SQL

Debe aprender a usar join explícito adecuado sintaxis. Una regla simple:nunca use comas en from cláusula. Siempre usar join explícito s:

SELECT A.order_number, A.header_id, B.line_id, B.quantity,
       C.hold_price_id, C.released_flag
FROM Table_A a JOIN
     Table_B b
     ON a.header_id = b.header_id JOIN
     Table_C c
     ON c.header_id = b.header_id AND c.line_id = b.line_id
WHERE a.order_number = '123';