Unreinforced masonry aggregate buildings, common in historic Mediterranean centers, often result from incremental additions that form large compounds of interconnected units. These units typically differ in geometry, materials, and boundary conditions, with uncertain connections that give rise to the "aggregate effect", a phenomenon where unit interactions significantly alter overall seismic performance. This study proposes an elementary three-unit model of quasi-identical structures with similar isolated behavior to investigate this effect under varying inter-unit connections. The setup enables direct comparison of unit responses within the aggregate, linking differences in strength, displacement, and damage to unit position and boundary conditions. Assuming a rigid diaphragm, three boundary conditions are explored: (i) rigid inter-unit connections, (ii) degrading connections, and (iii) isolated units. Fragility analysis is conducted using thirty ground motions, with collapse limit state (SLC) exceedance evaluated by two criteria: in-plane drift limits and numerical non-convergence. The model uses a homogenized material approach with 2D layered shell elements and a continuum interface strategy to represent degrading inter-unit behavior, capturing both in-plane and out-of-plane failure modes. Results highlight that stronger inter-unit connections improve the performance of individual units. The unit’s position within the building ensemble also significantly influences its response, with external units more vulnerable to seismic demands, while internal units benefit from confinement provided by adjacent structures. However, this study also demonstrates that, despite the simplicity of the model, relying on a single damage parameter may obscure other critical failure mechanisms. These limitations are expected to become more pronounced with the introduction of additional factors such as slab flexibility and geometric irregularities.