  Synchrotron radiation from a decelerating blastwave is a widely accepted
model of radio to X-ray afterglow emission from gamma-ray bursts (GRBs). GeV
gamma-ray emission detected by the Fermi Large Area Telescope (LAT) and the
duration of which extends beyond the prompt gamma-ray emission phase, is also
compatible with broad features of afterglow emission. We revisit the
synchrotron self-Compton (SSC) emission model from a decelerating blastwave to
fit multiwavelength data from three bright GRBs, namely GRB~190114C,
GRB~130427A and GRB~090510. We constrain the afterglow model parameters using
the simultaneous fit of the spectral energy distributions at different times
and light curves at different frequencies for these bursts. We find that a
constant density interstellar medium is favored for the short GRB~090510, while
a wind-type environment is favored for the long GRB~130427A and GRB~190114C.
The sub-TeV component in GRB~190114C detected by MAGIC is the SSC emission in
our modelling. Furthermore we find that the SSC emission in the Thomson regime
is adequate to fit the spectra and light curves of GRB~190114C. For the other
two GRBs, lacking sub-TeV detection, the SSC emissions are also modeled in the
Thomson regime. For the model parameters we have used, the $\gamma\gamma$
attenuation in the blastwave is negligible in the sub-TeV range compared to the
redshift-dependent $\gamma\gamma$ attenuation in the extragalactic background
light.
