A brief review on the thermodynamic properties of 3He films adsorbed on graphite is presented. The second solid layer of 3He adsorbed on graphite is a particularly interesting two-dimensional system of localized identical spin-1/2 fermions, which evolves from antiferromagnetism to ferromagnetism as a function of increasing density. This striking behavior is interpreted within a generalized Heisenberg model including competing ring-exchange interactions. While ferromagnetic three-particle exchange dominates at the highest densities, higher order antiferromagnetic ring-exchange processes become significant at low densities leading to highly frustrated antiferromagnetic or ferromagnetic phases. At low temperature, the frustrated antiferromagnetic and ferromagnetic states may correspond to unique examples of new quantum phases as suggested by recent theoretical results. The relevance of this ring-exchange model for other systems of localized fermions is also discussed.