  We present results on the dust attenuation of galaxies at redshift $\sim3-6$
by studying the relationship between the UV spectral slope ($\beta_{\rm UV}$)
and the infrared excess (IRX; $L_{\rm IR}$/$L_{\rm UV}$) using ALMA
far-infrared continuum observations. Our study is based on a sample of 67
massive, star-forming galaxies with a median mass of $M_{\ast}\sim
10^{10.7}\,M_{\rm \odot}$ spanning a redshift range $z=2.6-3.7$ (median
$z=3.2$) that were observed with ALMA at $\lambda_{rest}=300\,{\rm \mu m}$.
Both the individual ALMA detections (41 sources) and stacks including all
galaxies show the IRX-$\beta_{\rm UV}$ relationship at $z\sim3$ is mostly
consistent with that of local starburst galaxies on average. However, we find
evidence for a large dispersion around the mean relationship by up to $\pm0.5$
dex. Nevertheless, the locally calibrated dust correction factors based on the
IRX-$\beta_{\rm UV}$ relation are on average applicable to main-sequence
$z\sim3$ galaxies. This does not appear to be the case at even higher
redshifts, however. Using public ALMA observations of $z\sim4-6$ galaxies we
find evidence for a significant evolution in the IRX-$\beta_{\rm UV}$ and the
IRX-$M_{\ast}$ relations beyond $z\sim3$ toward lower IRX values. We discuss
several caveats that could affect these results, including the assumed dust
temperature. ALMA observations of larger $z>3$ galaxy samples will be required
to confirm this intriguing redshift evolution.
