Fresnel through multiple media

I am interested in the propagation of light through different media in the context of Fresnel diffraction equation,
$$
U_{image}(x,y,z) = U_{object}(x,y,0) circledast h(x,y,z)
$$
where $U_{object}(x,y,0)$ is the (scalar) field at the $z=0$ plane, $h(x,y,z)$ is the so called impulse response, and the symbol $circledast$ indicates a convolution.
If we consider the propagation of the field $U_{object}(x,y,0)$ through three media with refractive indices $n_i$ and lengths $z_i$

I know that we are allowed to break up the propagation into three parts: In each part we propagate the field by the optical distance $n_i cdot z_i$ — for $i=1, 2, 3$. Intuitively, I would argue that we obtain the same result, if we propagate the fields once by the optical distance $sum_i n_i z_i$. However, I expect that this is valid only if the diameter $D$ is "small" compared to the distances $z_i$, such that the rays are almost parallel. Furthermore, I would not be surprised, if there are other conditions, which must be met. Hence my question:

What are the needed conditions for simplifying the calculation and using a single step propagation? I am mainly interested in "every day setups", where the media are air, water and glass.