  We introduce a new statistic, skip, on rational $(3,n)$-Dyck paths and define
a marked rank word for each path when $n$ is not a multiple of 3. If a triple
of valid statistics (area,skip,dinv) are given, we have an algorithm to
construct the marked rank word corresponding to the triple. By considering all
valid triples we give an explicit formula for the $(m,n)$-rational
$q,t$-Catalan polynomials when $m=3$. Then there is a natural bijection on the
triples of statistics (area,skips,dinv) which exchanges the statistics area and
dinv while fixing the skip. Thus we prove the $q,t$-symmetry of
$(m,n)$-rational $q, t$-Catalan polynomials for $m=3$.
