  There is a well-established relation between the spatial asymmetry in the
initial stage of a heavy-ion collision and the final momentum anisotropy, which
allows for a separation of effects from initial conditions vs. later evolution
and has proved exceptionally powerful. However, until recently it has only been
studied in two dimensions -- either through boost-invariant simulations or
studying only quantities at mid-rapidity. We explore an extension to 3
dimensions, in order to determine whether a similar understanding can be
obtained for the rapidity dependence of the collision system. In particular, we
introduce rapidity-dependent eccentricities and investigate a trivial extension
of the 2D eccentricity scaling of elliptic and triangular flow, as well as a
way to systematically improve these initial-state estimators. We then explore
the dependence of the resulting response coefficients on shear viscosity and
initial total energy.
