  We study the multifrequency emission and spectral properties of the quasar 3C
279. We observed 3C 279 in very high energy (VHE, E>100GeV) gamma rays, with
the MAGIC telescopes during 2011, for the first time in stereoscopic mode. We
combine these measurements with observations at other energy bands: in high
energy (HE, E>100MeV) gamma rays from Fermi-LAT, in X-rays from RXTE, in the
optical from the KVA telescope and in the radio at 43GHz, 37GHz and 15GHz from
the VLBA, Mets\"ahovi and OVRO radio telescopes and optical polarisation
measurements from the KVA and Liverpool telescopes. During the MAGIC
observations (February to April 2011) 3C 279 was in a low state in optical,
X-ray and gamma rays. The MAGIC observations did not yield a significant
detection. These upper limits are in agreement with the extrapolation of the HE
gamma-ray spectrum, corrected for extragalactic background light absorption,
from Fermi-LAT. The second part of the MAGIC observations in 2011 was triggered
by a high activity state in the optical and gamma-ray bands. During the optical
outburst the optical electric vector position angle rotatated of about 180
degrees. There was no simultaneous rotation of the 43GHz radio polarisation
angle. No VHE gamma rays were detected by MAGIC, and the derived upper limits
suggest the presence of a spectral break or curvature between the Fermi-LAT and
MAGIC bands. The combined upper limits are the strongest derived to date for
the source at VHE and below the level of the previously detected flux by a
factor 2. Radiation models that include synchrotron and inverse Compton
emissions match the optical to gamma-ray data, assuming an emission component
inside the broad line region (BLR) responsible for the high-energy emission and
one outside the BLR and the infrared torus causing optical and low-energy
emission. We interpreted the optical polarisation with a bent trajectory model.

High Energy Astrophysical Phenomena;Astrophysics of Galaxies