We have measured heat capacities of monolayer 3He floated on a superfluid 4He thin film (three atomic layers) adsorbed on graphite at low temperatures. The 3He films behave as degenerate 2D Fermi fluids with m* ≈ 1.3m3, where m* is the quasiparticle effective mass and m3 is the bare mass of3He, in the whole temperature range we studied (1 ≤ T ≤ 80 mK). No anomalous behavior suggesting “puddling” nor other phase transitions is observed. In contrast to our previous measurements for pure 3He films without the underlying 4He film, the temperature independent heat-capacity contribution is not observed. This can be explained by the second-layer localized spins trapped on substrate heterogeneities being replaced by nonmagnetic 4He.