We present a wide-field (30 arcmin diameter) 850 μm Submillimetre Common-User Bolometer Array-2 map of the spectacular three-component merging supercluster, RCS 231953+00, at z = 0.9. The brightest submillimetre galaxy (SMG) in the field (S850 ≈ 12 mJy) is within 30 arcsec of one of the cluster cores (RCS 2319–C), and is likely to be a more distant, lensed galaxy. Interestingly, the wider field around RCS 2319–C reveals a local overdensity of SMGs, exceeding the average source density by a factor of 4.5, with a <1 per cent chance of being found in a random field. Utilizing Herschel observations from the Spectral and Photometric Imaging Receiver, we find that three of these SMGs have similar submillimetre colours. We fit their observed 250–850μm spectral energy distributions to estimate their redshift, yielding 2.5 < z < 3.5, and calculate prodigious star formation rates ranging from 500 to 2500 M⊙ yr−1. We speculate that these galaxies are either lensed SMGs, or signpost a physical structure at z ≈ 3: a ‘protocluster’ inhabited by young galaxies in a rapid phase of growth, destined to form the core of a massive galaxy cluster by z = 0.