We study the sandpile model on three-dimensional spanning Ising clusters with the temperature T treated as the control parameter. By analyzing the three dimensional avalanches and their twodimensional projections (which show scale-invariant behavior for all temperatures), we uncover two universality classes with different exponents (an ordinary BTW class, and SOCT =∞), along with a tricritical point (at Tc, the critical temperature of the host) between them. The SOCT =∞ universality class is characterized by the exponent of the avalanche size distribution τ T =∞ = 1.27 ± 0.03, consistent with the exponent of the size distribution of the Barkhausen avalanches in amorphous ferromagnets (Phys. Rev. L 84, 4705 (2000)). The tricritical point is characterized by its own critical exponents and also some additional scaling behavior in its vicinity. In addition to the avalanche exponents, some other quantities like the average height, the spanning avalanche probability (SAP) and the average coordination number of the Ising clusters change significantly the behavior at this point, and also exhibit power-law behavior in terms of ≡ T −Tc Tc , defining further critical exponents. Importantly the finite size analysis for the activity (number of topplings) per site shows the scaling behavior with exponents β = 0.19 ± 0.02 and ν = 0.75 ± 0.05. A similar behavior is also seen for the SAP and the average avalanche height. The fractal dimension of the external perimeter of the two-dimensional projections of avalanches is shown to be robust against T with the numerical value D f = 1.25 ± 0.01.In the context of out-of-equilibrium critical phenomena, self-organized critical (SOC) systems have attracted much attention because of their role in a wide range of systems, from finance [1] and biological [2] to granular matter [3], the brain [4] and neural networks in general [5]. SOC systems are characterized by their avalanche dynamics resulting from slow driving of the system. Vortex avalanche dynamics in type II superconductors [6], earthquakes [7], solar flares [8], microfracturing processes [9], fluid flow in porous media [10], phase transition-like behavior of the magnetosphere [11], bursts in filters [12], phase transitions in jammed granular matter [3], and avalanches dynamics in the rat cortex [13] are some natural examples for SOC. This large class of natural systems inspired theoretical models with the aim of capturing the dominant internal dynamics that causes the avalanches.Here we find evidence for a novel non-equilibrium universality class, and propose a new type of outof-equilibrium phase transition between SOC models induced by the geometry of the underlying graph upon which the model is defined. It might be applicable to experiments with spatial flow patterns of transport in heterogeneous porous media [14], which involve the toppling of fluid [15]. Another example is the Barkhausen effect in magnetic systems [16], for which the avalanches have been shown to exhibit scaling behavior with an avalanche size exponent 1.27 ± 0....