  We analyse 870um Atacama Large Millimetre Array (ALMA) dust continuum
detections of 41 canonically-selected z~3 Lyman-break galaxies (LBGs), as well
as 209 ALMA-undetected LBGs, in follow-up of SCUBA-2 mapping of the UKIDSS
Ultra Deep Survey (UDS) field. We find that our ALMA-bright LBGs lie
significantly off the locally calibrated IRX-beta relation and tend to have
relatively bluer rest-frame UV slopes (as parametrised by beta), given their
high values of the 'infrared excess' (IRX=L_IR/L_UV), relative to the average
'local' IRX-beta relation. We attribute this finding in part to the young ages
of the underlying stellar populations but we find that the main reason behind
the unusually blue UV slopes are the relatively shallow slopes of the
corresponding dust attenuation curves. We show that, when stellar masses are
being established via SED fitting, it is absolutely crucial to allow the
attenuation curves to vary (rather than fixing it on Calzetti-like law), where
we find that the inappropriate curves may underestimate the resulting stellar
masses by a factor of ~2-3x on average. In addition, we find these LBGs to have
relatively high specific star-formation rates (sSFRs), dominated by the dust
component, as quantified via the fraction of obscured star formation ( f_obs =
SFR_IR/SFR_(UV+IR)). We conclude that the ALMA-bright LBGs are, by selection,
massive galaxies undergoing a burst of a star formation (large sSFRs, driven,
for example, by secular or merger processes), with a likely geometrical
disconnection of the dust and stars, responsible for producing shallow dust
attenuation curves.
