  Magnetic field of accreting neutron stars determines their overall behaviour
including the maximum possible luminosity. Some models require an above-average
magnetic field strength (> 10^13 G) in order to explain super-Eddington mass
accretion rate in the recently discovered class of pulsating ultraluminous
X-ray sources (ULX). The peak luminosity of SMC X-3 during its major outburst
in 2016-2017 reached ~2.5x10^39 erg/s comparable to that in ULXs thus making
this source the nearest ULX-pulsar. SMC X-3 belongs to the class of transient
X-ray pulsars with Be optical companions, and exhibited a giant outburst in
July 2016 - February 2017. The source has been observed during the entire
outburst with the Swift/XRT and Fermi/GBM telescopes, as well as the NuSTAR
observatory. Collected data allowed us to estimate the magnetic field strength
of the neutron star in SMC X-3 using several independent methods. Spin
evolution of the source during and between the outbursts and the luminosity of
the transition to so-called propeller regime in the range of (0.3 - 7)x10^35
erg/s imply relatively weak dipole field of (1 - 5)x10^12 G. On the other hand,
there is also evidence for much stronger field in the immediate vicinity of the
neutron star surface. In particular, transition from super- to sub-critical
accretion regime associated with cease of the accretion column, absence of
cyclotron absorption features in the broadband X-ray spectrum of the source
obtained with NuSTAR and very high peak luminosity favor an order of magnitude
stronger field. This discrepancy makes SMC X-3 a good candidate to posses
significant non-dipolar components of the field, and an intermediate source
between classical X-ray pulsars and accreting magnetars which may constitute an
appreciable fraction of ULX population.
