  The triangular flow in ultrarelativistic $^3$He-Au collisions at RHIC
energies is enhanced due to the triangular arrangement of the nucleon
configurations in $^3$He. We study the fireball eccentricities in the Glauber
Monte Carlo model and find that since the configurations of the projectile
$^3$He are elongated triangles, the created fireball has a large ellipticity
and a smaller triangularity. The dependence of the triangularity on centrality
is weak, so it cannot be extracted from the centrality dependence of the
triangular flow $v_3$, as it is dominated by the centrality dependence of the
hydrodynamic response. We propose to look at the centrality dependence of the
ratio $v_n\{4\}/v_n\{2\}$, where the uncertainties from the hydrodynamic
response cancel, and show that the basic signature of the geometry-driven
collective flow is the raise of the ratio $v_3\{4\}/v_3\{2\}$ with the number
of participant nucleons for centralities less than 10%.
