  This paper presents a defense approach to safeguard a protected area against
an attack by a swarm of adversarial agents in three-dimensional (3D) space. We
extend our 2D `StringNet Herding' approach, in which a closed formation of
string-barriers is established around the adversarial swarm to confine their
motion and herd them to a safe area, to 3D spaces by introducing 3D-StringNet.
3D-StringNet is a closed 3D formation of triangular net-like barriers. We
provide a systematic approach to generate three types of 3D formations that are
used in the 3D herding process and modifications to the finite-time convergent
control laws developed in \cite{chipade2020swarmherding} that are required for
a 3D environment. Furthermore, for given initial positions of the defenders, we
provide conditions on the initial positions of the attackers for which the
defenders are guaranteed to gather as a specified formation at a position on
the shortest path of the attackers to the protected area before attackers reach
there.
