Demand Response (DR) is transitioning from traditional event-based DR to continuous demand flexibility (DF). This transformation is bringing about changes in several dimensions of building-grid interaction: from static DR events on a few days per year to continuous flexibility every hour of every day, from a few customers who sign up for DR programs to all utility customers, and from a few devices such as a smart thermostat to all customer devices. This paper presents a system architecture that enables that transformation and explains how all of the above dimensions are facilitated by OpenADR 3.0. It also describes the nature of the implementation details to incorporate OpenADR 3.0 functionality into building loads, through building gateways, and other devices.