  The dynamical evolution of triple stellar systems could induce the formation
of compact binaries and binary mergers. Common envelope (CE) evolution, which
plays a major role in the evolution of compact binary systems, can similarly
play a key role in the evolution of triples. Here we use hydrodynamical
simulations coupled with few-body dynamics to provide the first detailed models
of triple common envelope (TCE) evolution. We focus on the circumstellar case,
where the envelope of an evolved giant engulfs a compact binary orbiting the
giant, which then in-spirals into the core of the evolved star. Through our
exploratory modeling we find several possible outcomes of such TCE: (1) The
merger of the binary inside the third star's envelope; (2) The disruption of
the in-spiraling binary following its plunge, leading to a chaotic triple
dynamics of the stellar-core and the two components of the former disrupted
binary. The chaotic evolution typically leads to the in-spiral and merger of at
least one of the former binary components with the core, and sometimes to the
ejection of the second, or alternatively its further now-binary common-envelope
evolution. The in-spiral in TCE leads to overall slower in-spiral, larger mass
ejection and the production of more aspherical remnant, compared with a
corresponding binary case of similar masses, due to the energy/momentum
extraction from the inner-binary. We expect TCE to play a key role in producing
various types of stellar-mergers and unique compact binary systems, and
potentially induce transient electromagnetic and gravitational-wave sources.
