A single-homed full-mesh design is the best match when each WAN router has only one WAN interface, no additional WAN links can be purchased, and the customer still wants resilient site-to-site reachability. Single-homed means each site has one physical WAN attachment. Full mesh describes the logical connectivity model between sites, where each site can communicate directly with the others across the WAN service or overlay rather than relying completely on a central hub for intersite traffic. A hub-and-spoke design is simpler, but it creates dependency on the hub for spoke-to-spoke communication and provides poorer resiliency for a three-site design. Dual-homed hub-and-spoke and dual-homed full mesh both require additional WAN connectivity or interfaces, which violates the stated budget and hardware constraints. The best available design is therefore to use the single WAN attachment at each site while building a full-mesh logical topology over the provider or overlay service. Reference topics: WAN topology design, single-homed connectivity, full mesh, hub-and-spoke limitations, high availability under interface constraints.