It is possible for a relation to be in 3NF and have only one candidate key and still not be in BCNF. In order for a relation to be in BCNF, every determinant in the relation must be a candidate key. In other words, if there are any non-trivial functional dependencies in the relation, at least one of the determinants must be a candidate key.
For example, consider the relation R(A, B, C, D) with the functional dependencies A -> B, B -> C, and C -> D. This relation is in 3NF because no non-prime attribute is transitively dependent on any candidate key. However, it is not in BCNF because the determinant C -> D is not a candidate key.
In general, it is not true that a relation in 3NF with only one candidate key is automatically in BCNF.