We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in the period 2007 July-2009 January. During the 2008 May-2009 January period, the source average flux was highly variable, with a clear fading trend toward the end of the period, from an average {gamma}-ray flux F_E>100MeV_>~200x10^-8^photons/cm^2^/s in 2008 May-June, to F_E>100MeV_~80x10^-8^photons/cm^-2^/s in 2008 October-2009 January. In 2007 July-August and 2008 May-June, 3C 454.3 was monitored by Rossi X-ray Timing Explorer (RXTE). We also carried out simultaneous Swift observations during all AGILE campaigns. Swift/XRT detected 3C 454.3 with an observed flux in the 2-10keV energy band in the range (0.9-7.5)x10^-11^erg/cm^2^/s and a photon index in the range {Gamma}XRT=1.33-2.04. In the 15-150keV energy band, when detected, the source has an average flux of about 5mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period in the radio, millimeter, near-IR, and optical bands. The observations show an extremely variable behavior at all frequencies, with flux peaks almost simultaneous with those at higher energies. An analysis of 15GHz and 43GHz VLBI core radio flux observations in the period 2007 July-2009 February shows an increasing trend of the core radio flux, anti-correlated with the higher frequency data, allowing us to derive the value of the source magnetic field.