In this paper, we present the Massive Mobile Multiuser (M³) framework, a software platform designed to enable setup-free, real-time, concurrent interaction with shared public displays through large numbers of personal mobile devices. This work is motivated by the fact that simultaneous interaction of multiple persons with public displays requires either dedicated tracking hardware to detect gestures or touch, or a way for users to interact through their personal mobile devices. The latter option provides more flexibility but also presents a heightened entry barrier as it often requires installation of custom software. To address these issues, M³ enables immediate interaction through the mobile browser without requiring prior setup on the user side, and real-time interaction suitable for fast multiplayer games. We present a detailed analysis of latency sources and findings from two real-world deployments of our framework in public settings with up to 17 concurrent users. Despite a resource-constrained environment and an unpredictable selection of client devices, M³ consistently delivers performance suitable for real-time interaction.