Can PostgreSQL use multi-column index for FK constraint validation?

Hello,

If I have a primary key index of the form:
(col1, col2, col3)
and a foreign key constraint of the form:
FOREIGN KEY (col1, col2) REFERENCES foo
ON DELETE CASCADE ON UPDATE CASCADE
should I create a separate index (col1, col2) or is PostgreSQL capable of
using the primary key's index?

Thanks,

Dane