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The dominant mechanism leading to the formation of brown dwarfs (BDs) remainsuncertain. The most direct keys to formation, which are obtained from youngerobjects (pre-BD cores and proto-BDs), are limited by the very low numberstatistics available. We aim to identify and characterize a set of pre- andproto-BDs as well as Class II BDs in the Lupus 1 and 3 molecular clouds to testtheir formation mechanism. We performed ALMA band 6 (1.3 mm) continuumobservations of a selection of 64 cores previously identified from AzTEC/ASTEdata (1.1 mm), along with previously known Class II BDs in the Lupus 1 and 3molecular clouds. Surveyed archival data in the optical were used to complementthese observations. We expect these ALMA observations prove efficient indetecting the youngest sources in these regions, since they probe the frequencydomain at which these sources emit most of their radiation. We detected 19sources from 15 ALMA fields. Considering all the pointings in our observingsetup, the ALMA detection rate was $\sim$23% and the derived masses of thedetected sources were between $\sim$0.18 and 124 $\mathrm{M_{Jup}}$. Weclassified these sources according to their spectral energy distribution as 5Class II sources, 2 new Class I/0 candidats, and 12 new possible pre-BD ordeeply embedded protostellar candidates. We detected a promising candidate fora Class 0/I proto-BD source and inferred the disk dust mass of a bona fideClass II BD. The pre-BD cores might be the byproduct of an ongoing process oflarge-scale collapse. The Class II BD disks follow the correlation between diskmass and the mass of the central object that is observed at the low-massstellar regime. We conclude that it is highly probable that the sources in thesample are formed as a scaled-down version of low-mass star formation, althoughdisk fragmentation may be responsible for a considerable fraction of BDs.

ALMA observations of the early stages of substellar formation in the Lupus 1 and 3 molecular clouds