  We study infinite words coding an orbit under an exchange of three intervals
which have full complexity $\C(n)=2n+1$ for all $n\in\N$ (non-degenerate 3iet
words). In terms of parameters of the interval exchange and the starting point
of the orbit we characterize those 3iet words which are invariant under a
primitive substitution. Thus, we generalize the result recently obtained for
sturmian words.
