  We present multiwavelength observations of 3C 454.3 from April 2008 to March
2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray
data from Swift, and gamma-ray data from the AGILE and Fermi satellites. We
improved the calibration of optical-UV data from the UVOT and OM instruments
and estimated the Lyalpha flux to disentangle the contributions from different
components in this spectral region. The observations reveal prominent
variability above 8 GHz. In the optical-UV band, the variability amplitude
decreases with increasing frequency due to a steadier radiation from both a
broad line region and an accretion disc. The optical flux reaches nearly the
same levels in the 2008-2009 and 2009-2010 observing seasons; the mm one shows
similar behaviour, whereas the gamma and X-ray flux levels rise in the second
period. Two prominent gamma-ray flares in mid 2008 and late 2009 show a
double-peaked structure, with a variable gamma/optical flux ratio. The X-ray
flux variations seem to follow the gamma-ray and optical ones by about 0.5 and
1 d, respectively. We interpret the multifrequency behaviour in terms of an
inhomogeneous curved jet, where synchrotron radiation of increasing wavelength
is produced in progressively outer and wider jet regions, which can change
their orientation in time. In particular, we assume that the long-term
variability is due to this geometrical effect. By combining the optical and mm
light curves to fit the gamma and X-ray ones, we find that the gamma (X-ray)
emission may be explained by inverse-Comptonisation of synchrotron optical (IR)
photons by their parent relativistic electrons (SSC process). A slight,
variable misalignment between the synchrotron and Comptonisation zones would
explain the increased gamma and X-ray flux levels in 2009-2010, as well as the
change in the gamma/optical flux ratio during the outbursts peaks.
