We present the concept of a shallow, single-layer, multi-PMT water Cherenkov detector (WCD) unit for ground-based gamma-ray astronomy. The design was developed as part of the R&D activities for SWGO, as a candidate WCD unit solution for the future Observatory. The main objective of the proposal is to achieve a low-volume single-layer surface detection unit with good calorimetry and timing of the shower front, and excellent gamma-hadron discrimination capability. The unit name, Mercedes, is derived from its configuration, where 3 PMTs are positioned equidistant from the center of the unit in a 120-deg symmetric azimuthal arrangement. Background rejection, in the TeV range, is based on the asymmetric illumination of the PMTs by energetic muons traversing the water volume. The mechanical implementation is based on rotomolded tanks similar to those used in the Pierre Auger Observatory. For easy deployment, the PMTs may be installed through openings in the lateral walls of the tank, and placed outside water, observing the calorimetric volume through transparent windows in the tank bladders. The highly innovative tank proposal, whose production technique is already patented, and at the commercial stage, has a jacketed- wall structure that houses all electronics and cabling, and a layer of insulating foam to avoid freezing at high-altitude. The mechanical concept was conceived for inexpensive production, easy deployment and maintenance, and to provide a cost-effective solution for high-altitude large array designs with several thousand units, such as in SWGO.