  Boson Sampling is a computational paradigm representing one of the most
viable and pursued approaches to demonstrate the regime of quantum advantage.
Recent results have demonstrated significant technological leaps in
single-photon generation and detection, leading to progressively larger
experimental instances of Boson Sampling experiments in different photonic
systems. However, a crucial requirement for a fully-fledged platform solving
this problem is the capability of implementing large-scale interferometers,
that must simultaneously exhibit low losses, high degree of reconfigurability
and the realization of arbitrary transformations. In this work, we move a step
forward in this direction by demonstrating the adoption of a compact and
reconfigurable 3D-integrated platform for photonic Boson Sampling. We perform
3- and 4-photon experiments by using such platform, showing the possibility of
programming the circuit to implement a large number of unitary transformations.
These results show that such compact and highly-reconfigurable layout can be
scaled up to experiments with larger number of photons and modes, and can
provide a viable direction for hybrid computing with photonic processors.
