  The formation and early evolution of circumstellar discs often occurs within
dense, newborn stellar clusters. For the first time, we apply the moving-mesh
code AREPO, to circumstellar discs in 3-D, focusing on disc-disc interactions
that result from stellar fly-bys. Although a small fraction of stars are
expected to undergo close approaches, the outcomes of the most violent
encounters might leave an imprint on the discs and host stars that will
influence both their orbits and their ability to form planets. We first
construct well-behaved 3-D models of self-gravitating discs, and then create a
suite of numerical experiments of parabolic encounters, exploring the effects
of pericenter separation r_p, disc orientation and disc-star mass ratio
(M_d/M_*) on the orbital evolution of the host stars. Close encounters (2r_p<~
disc radius) can truncate discs on very short time scales. If discs are
massive, close encounters facilitate enough orbital angular momentum extraction
to induce stellar capture. We find that for realistic primordial disc masses
M_d<~0.1M_*, non-colliding encounters induce minor orbital changes, which is
consistent with analytic calculations of encounters in the linear regime. The
same disc masses produce entirely different results for grazing/colliding
encounters. In the latter case, rapidly cooling discs lose orbital energy by
radiating away the energy excess of the shock-heated gas, thus causing capture
of the host stars into a bound orbit. In rare cases, a tight binary with a
circumbinary disc forms as a result of this encounter.
