(u × v) • w makes sense, since u × v is another vector, and so (u × v) • w is a scalar.
u × (v • w) does not make sense, since v • w is a scalar, and the cross product is not defined between a vector and a scalar.
(u × v) × w makes sense, since u × v is a vector and so is w.
u • (v • w) does not make sense, since v • w is scalar, and the dot product is not defined between a vector and a scalar.