Попробуйте так:
X++:
qbdsA = query.addDataSource(tablenum(A));
qbdsA.addRange(fieldnum(A, ItemId));
qbds = qbdsA.addDataSource(tablenum(B));
qbds.relations(true);
qbds.fetchMode(QueryFetchMode::One2One);
qbds = qbds.addDataSource(tablenum(C));
qbds.addLink(fieldnum(A, K), fieldnum(C, K), qbdsA.name())
qbds.fetchMode(QueryFetchMode::One2One);