A non-linear model of major fire spread in a tunnel is described briefly. It employs the concepts of bifurcation theory and identifies the onset of instability with major fire spread in a tunnel. In particular, the model associates the existence of a fold bifurcation with dramatic fire spread. The purpose is to identify the thermo-physical and geometrical conditions which lead to instability and sudden fire spread. The particular case considered assumes the existence of a longitudinal forced ventilation and predicts the critical heat release rate needed for a fire to spread from an initial fire to a target object. The illustrative case of fire spread from an initial fire to a heavy goods vehicle (HGV) is presented; it is not restricted to this case, however. The model is identified with the name FIRE-SPRINT A3, which is an acronym of Fire Spread in Tunnels, Model A, Version 3. It is a development of an earlier model, FIRE-SPRINT A2. The current model takes greater account of thermal radiation than was done in earlier work and also assumes a more plausible flame shape.