  The long-term optical, X-ray and $\gamma$-ray data of blazar 3C 279 have been
compiled from $Swift$-XRT, $RXTE$ PCA, $Fermi$-LAT, SMARTS and literature. The
source exhibits strong variability on long time scales. Since 1980s to now, the
optical $R$ band light curve spans above 32 yr, and a possible 5.6-yr-long
quasi-periodic variation component has been found in it. The optical spectral
behavior has been investigated. In the optical band, the mean spectral index is
-1.71. The source exhibits an obvious special spectral behavior. In the low
state, the source shows a clear bluer-when-brighter behavior in a sense that
the optical spectrum turns harder (flatter) when the brightness increases.
While in the high state, the optical spectrum is stable, that means the source
spectral index does not vary with the brightness. The correlation analysis has
been performed among optical, X-ray and $\gamma$-ray energy bands. The result
indicates that the variations of $\gamma$-ray and X-ray bands are well
correlated without time delay on the time scale of days, and their variations
exhibit weak correlations with those of optical band. The variations, especial
outbursts, are simultaneous, but the magnitude of variations is
disproportionate. The detailed analysis reveals that the main outbursts exhibit
strong correlations in different $\gamma$-ray, X-ray and optical bands.
