To create a phase diagram of two-dimensional (2D) 3He fluids, we measured heat capacity of the 3He adatoms down to 20mK as a function of the density. The 3He adatoms are deposited on hectorite precoated with 4He in the amount of 24.70μmol/m2, just above the onset density for the 4He super-fluid. Isotherms of the heat capacity steeply increase with the 3He density up to nc = 2.4μmol/m2. Above nc, the heat capacity shows properties of the 2D Fermi liquids. We pointed out that the density dependence of the heat capacity is similar to those of the insulator-metal (I-M) transitions of some electron systems, e.g. Si:P. Assuming the transition from a localized state to the Fermi liquid at the density nc, the diameter of a localization area a*H was obtained as about 3.8Å using the Mott relation that the mean distance equals 2.2a*H at the transition. Effective mass and 2D spin fluctuations of the Fermi liquids are enhanced with the density by increasing correlations in the 2D liquids.