  The flat-spectrum radio quasar 3C~454.3 throughout the years has presented
very high activity phases (flares) in which the different wavebands increase
their flux dramatically. In this work, we perform multiwavelength analysis from
radio to gamma-rays and study the Mg~II~$\lambda 2798$\AA\ emission line and
the UV~Fe~II band from 2008-2018. We found that an increase in the 43 GHz flux
density of the quasi-stationary component C, coincides with the estimated time
at which a superluminal blob ejected from the radio core (which caused the
brightest flare of 2010) collides with the quasi-stationary component (at a
projected distance of $\sim4.6$ pc from the radio core). The spectral index
different behavior in the first ($5000 < \text{JD}-2450000 < 5600$) and second
($6600 < \text{JD}-2450000 < 7900$) flaring periods suggest changes in the
physical conditions. The complex nature of the second period can be a result of
a superposition of multiple events at different locations. The Mg~II has an
anti-correlation with the UV-continuum while Fe~II correlates positively.
Except by the time of the brightest flare of 2010, when both have a strong
response at high continuum luminosities. Our results suggest that the dominant
gamma-ray emission mechanism for the first flaring period is External Compton.
For the second flaring period the seed photons emission region is co-spatial
with the gamma-ray emission region. However, a SED study using a multizone jet
emission model is required to confirm the nature of each significant flare
during the second period.
