  We discuss theoretically static and dynamical properties of $XY$ and
Heisenberg antiferromagnets on triangular lattice with random vacancies and
substitutional spins. It is shown that the distortion of $120^\circ$ magnetic
order produced by a single defect is described by electrostatic equations for a
field of an electrically neutral complex of six charges located around the
impurity. The first finite term in the multipole expansion of this field is the
octupole moment which decays as $1/r^3$ with the distance $r$. The linearity of
equations allows to describe analytically the distortion of the long-range
magnetic order at a small concentration $c$ of defects. We obtain analytically
renormalization of the elastic neutron scattering cross section and the magnon
spectrum $\epsilon_{\bf k}$ in the leading order in $c$. We find that the
scattering on impurities renormalizes weakly the bare spectrum $\epsilon_{\bf
k}\propto k$ at $k\gg\sqrt c$. However the renormalization is substantial of
the long-wavelength magnon spectrum at $k\ll\sqrt c$: $\epsilon_{\bf k}\propto
\sqrt{c /\ln(1/k)}$ at $k\to0$ and there is a parametrically large region in
which magnons with not too small momenta are overdamped and localized. This
strong modification of the long-wavelength spectrum leads to the stabilization
of the slightly distorted magnetic long-range order at $T<T_N\sim
S^2J/\ln(1/c)$ and to the considerable change in the density of states and in
the specific heat. The overdamped modes arise also in quasi-2D spin systems on
a stacked triangular lattice.
