  The Be X-ray pulsar SMC X-3 underwent an extra long and ultraluminous giant
outburst from 2016 August to 2017 March. The peak X-ray luminosity is up to
$\sim 10^{39}$ erg/s, suggesting a mildly super-Eddington accretion onto the
strongly magnetized neutron star. It therefore bridges the gap between the
Galactic Be/X-ray binaries ($L_{\rm X}^{\rm peak} \leq 10^{38}$ erg/s) and the
ultraluminous X-ray pulsars ($L_{\rm X}^{\rm peak} \geq 10^{40}$ erg/s) found
in nearby galaxies. A number of observations were carried out to observe the
outburst. In this paper, we perform a comprehensive phase-resolved analysis on
the high quality data obtained with the Nustar and XMM-Newton, which were
observed at a high and intermediate luminosity levels. In order to get a better
understanding on the evolution of the whole extreme burst, we take the Swift
results at the low luminosity state into account as well. At the early stage of
outburst, the source shows a double-peak pulse profile, the second main peak
approaches the first one and merges into the single peak at the low luminosity.
The second main peak vanishes beyond 20 keV, and its radiation becomes much
softer than that of the first main peak. The line widths of fluorescent iron
line vary dramatically with phases, indicating a complicated geometry of
accretion flows. In contrast to the case at low luminosity, the pulse fraction
increases with the photon energy. The significant small pulse fraction detected
below 1 keV can be interpreted as the existence of an additional thermal
component located at far away from the central neutron star.
