  We study zero--temperature properties of the 3d Edwards--Anderson Ising spin
glass on finite lattices up to size $12^3$. Using multicanonical sampling we
generate large numbers of groundstate configurations in thermal equilibrium.
Finite size scaling with a zero--temperature scaling exponent $y = 0.74 \pm
0.12$ describes the data well. Alternatively, a descriptions in terms of Parisi
mean field behaviour is still possible. The two scenarios give significantly
different predictions on lattices of size $\ge 12^3$.
