  The results of a long-term multiwavelength study of the powerful flat
spectrum radio quasar 3C 454.3 using {\it Fermi}-LAT and Swift XRT/UVOT data
are reported. In the $\gamma$-ray band, {\it Fermi}-LAT observations show
several major flares when the source flux was $>10^{-5}\:{\rm
photon\:cm^{-2}\:s^{-1}}$; the peak $\gamma$-ray flux above $141.6$ MeV,
$(9.22\pm1.96)\times10^{-5}\:{\rm photon\:cm^{-2}\:s^{-1}}$ observed on MJD
55519.33, corresponds to $2.15\times10^{50}\:{\rm erg\:s^{-1}}$ isotropic
$\gamma$-ray luminosity. The analysis of Swift XRT and UVOT data revealed a
flux increase, although with smaller amplitudes, also in the X-ray and
optical/UV bands. The X-ray emission of 3C 454.3 is with a hard spectral index
of $\Gamma_{\rm X}=1.16-1.75$, and the flux in the flaring states increased up
to $(1.80\pm0.18)\times10^{-10}{\rm erg\:cm^{-2}\: s^{-1}}$. Through combining
the analyzed data, it was possible to assemble 362 high-quality and
quasi-simultaneous spectral energy distributions of 3C 454.3 in 2008-2018 which
all were modeled within a one-zone leptonic scenario assuming the emission
region is within the broad line region, involving synchrotron, synchrotron
self-Compton and external Compton mechanisms. Such an extensive modeling is the
key for constraining the underlying emission mechanisms in the 3C 454.3 jet and
allows to derive the physical parameters of the jet and investigate their
evolution in time. The modeling suggests that during the flares, along with the
variation of emitting electron parameters, the Doppler boosting factor
increased substantially implying that the emission in these periods has most
likely originated in a faster moving region.
