We present an analysis of the dust attenuation of star forming galaxies at $z=2.5-4.0$ through the relationship between the UV spectral slope ($beta$), stellar mass ($M_{ast}$) and the infrared excess (IRX$=L_{rm{IR}}/L_{rm{UV}}$) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A$^3$COSMOS team, which includes an unprecedented sample of $sim1500$ galaxies at $zsim3$ as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $rm{mu Jy/beam}$ (1$sigma$). The detection rate is highly mass dependent, decreasing drastically below $log (M_{ast}/M_{odot})=10.5$. The detected galaxies show that the IRX-$beta$ relationship of massive ($log M_{ast}/M_{odot} > 10$) main sequence galaxies at $z=2.5-4.0$ is consistent with that of local galaxies, while starbursts are generally offset by $sim0.5,{rm dex}$ to larger IRX values. At the low mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX-$M_{ast}$ relation at $rm{log,(M_{ast}/M_{odot})>9}$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at $log M_{ast}/M_{odot}<10$. However, our results are consistent with early measurements at $zsim5.5$, indicating a potential redshift evolution between $zsim2$ and $zsim6$. Deeper observations targeting low mass galaxies will be required to confirm this finding.
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