jmc

We proceed by counting the complement, or the number of invalid 3-word sentences. A sentence is invalid precisely when it is of the form ``(word) splargh glumph'' or ``splargh glumph (word).'' There are 3 choices for the missing word in each sentence, and since each case is exclusive, we have a total of 6 invalid sentences. Since there are $3\cdot 3\cdot 3=27$ possible 3-word sentences with no restrictions, there are $27-6=\boxed{21}$ that satisfy the restrictions of the problem.