The increasing use of unmanned aerial vehicles (UAVs) in various commercial applications, such as precision agriculture and aerial remote sensing, is fast contributing to a significant growth in the UAV market. Also, it is crucial to provide continuous coverage after failures of wireless network components or additional bandwidth in high traffic situations. By introducing the concept of UAVs as a service (UaaS), we propose a novel framework, dubbed D³S, consisting of four phases: Demand, decision, deployment, and service. The main objective of this framework is to provide a realistic and streamlined approach to support the implementation of the UaaS paradigm. The technical problems involved include determining the type and number of UAVs to be deployed and their final locations (e.g., hovering or on-ground). They also include the trajectory planning, possibly several times, between charging stations and deployment locations. We present the application of the D³S framework to two case studies with the goal of providing wireless connectivity services to (i) static users after failures of wireless network components, including long-term and short-term failures, and (ii) dynamic users in wireless relaying systems.