Submillimetre galaxies (SMGs) are special amongst the population of dusty star forming galaxies, as they are bright in the submillimetre regime, but faint in UV to optical wavelengths. Their characteristic emission behaviour is the cumulative result of their complex history of gas accretion, star formation, and dust production. The recent advent of JWST now provides us with the unique opportunity to study the population of SMGs in unprecedented detail. However, SMGs are classically identified through observations with single-dish telescopes with large beam sizes, limited by source confusion. Thus, future JWST characterisation efforts based primarily on these classical catalogues will still be hampered by the well-known problem of identifying the optical counterparts of SMGs, ultimately hindering our efforts to gain a holistic understanding of the population. In this poster, I will present the A3COSMOS project: the largest ALMA archival effort to date, collecting all public ALMA archive data in the COSMOS and (as of recently) GOODS-South extragalactic fields and processing them into (sub)millimetre continuum images and source catalogues. Our database has a total areal coverage of over 1000arcmin², combined with the resolution and sensitivity of the ALMA interferometer, thus bridging the gap between wide single-dish surveys and deep pencil-beam surveys. This unique database provides the largest database of SMGs to date with a typical positional accuracy of <1'', allowing the direct identification of optical counterparts. This unique database can also be used to infer submillimetre number counts across multiple ALMA bands. However, because the database is not an ideal blind survey (i.e. a heterogeneous combination of targeted single ALMA pointings), such inference poses special challenges. In the rest of this poster, I will present the method we constructed to largely overcome these observational biases, a method which was tested and calibrated through simulations and then used to infer galaxy number counts in four ALMA receiver bands, spanning in wavelength from 0.8 to 3.6mm. These are the first homogeneously derived multi-wavelength number counts over such a large area. The number counts can be used to constrain theoretical models and simulations at multiple (sub)millimetre wavelengths. They are thus a useful tool in furthering our understanding of the population of SMGs, their properties, and evolution. These results also demonstrate the richness and diverse applicability of the A3COSMOS database for the study of these galaxies. It will complement present and future JWST deep-wide surveys at shorter wavelengths, enabling a more holistic view on this population.