  Charged-pion-interferometry measurements were made with respect to the
2$^{\rm nd}$- and 3$^{\rm rd}$-order event plane for Au$+$Au collisions at
$\sqrt{s_{_{NN}}}=200$ GeV. A strong azimuthal-angle dependence of the
extracted Gaussian-source radii was observed with respect to both the 2$^{\rm
nd}$- and 3$^{\rm rd}$-order event planes. The results for the 2$^{\rm
nd}$-order dependence indicate that the initial eccentricity is reduced during
the medium evolution, but not reversed in the final state, which is consistent
with previous results. In contrast, the results for the 3$^{\rm rd}$-order
dependence indicate that the initial triangular shape is significantly reduced
and potentially reversed by the end of the medium evolution, and that the
3$^{\rm rd}$-order oscillations are largely dominated by the dynamical effects
from triangular flow.
