The only way you can do this with plain Hibernate is via HQL. Something like the following would do

SELECT a.value, b.value, c.value
FROM A a
LEFT JOIN C c ON c.value = 'a_1' AND EXISTS(SELECT 1 FROM B b WHERE c.b = b AND b.a = a)
LEFT JOIN B b ON c.b = b AND b.a = a

If you are willing to try out something you could also give the JPA Criteria implementation of Blaze-Persistence a shot which provides support for entity joins.
It's a different implementation of the JPA Criteria API that in the end translates to JPQL/HQL that runs on an existing JPA provider. It also adds support for CTEs which would probably also solve your problem.