  We present the first unrestricted, three-dimensional relativistic
hydrodynamical calculations of the blob of gas associated with the jet
producing a gamma-ray burst. We investigate the deceleration phase of the blob
corresponding to the time when afterglow radiation is produced, concentrating
on the transition in which the relativistic beaming 1/gamma goes from being
less than theta, where gamma is the bulk Lorentz factor and theta is the
angular width of the jet, to 1/gamma > theta. We study the time dependent
evolution of the physical parameters associated with the jet, both parallel to
the direction of motion and perpendicular to it. We calculate light curves for
observers at varying angles with respect to the velocity vector of the blob,
assuming optically thin emission that scales with the local pressure. Our main
findings are that (i) gas ahead of the advancing blob does not accrete onto and
merge with the blob material but rather flows around the blob, (ii) the decay
light curve steepens at a time corresponding roughly to 1/gamma equals theta
(in accord with earlier studies), and (iii) the rate of decrease of the forward
component of momentum in the blob is well-fit by a simple model in which the
gas in front of the blob exerts a drag force on the blob, and the cross
sectional area of the blob increases quadratically with laboratory time (or
distance).
