  In three-dimensional AdS space, we consider the gravitational collapse of
dust shell and then investigate the quantum radiation from the collapsing shell
by employing the functional Schr\"odinger formalism. In the formation of the
BTZ black hole, the interior geometry of the shell can be chosen as either the
massless black hole or the global AdS space. In the incipient black hole limit,
we obtain the wave function exactly from the time-dependent Schr\"odinger
equation for a massless scalar field. Then, we show that the occupation number
of excited states can be written by analytic expressions, and the radiation
temperature is in agreement with the Hawking temperature, irrespective of the
specific choice of the interior geometries.
