Good guess on the He3 part... In their case, it is used as a fuel for a type of fusion, which directly creates electrical power from the reaction. It also has other uses, it can be used to power types of engines, except it does not chemically 'burn' like you're used to thiking, but again in sustaining a fusion reaction, it 'burns' with something on the order of 80% of the entropy of a hydrogen fusion reaction, except with BENEFITS - namely it is aneutronic, meaning it's 'exhaust' is all protons which are not harmful in the same way that neutron radiation is, and can be easily controlled with a simple magnetic field.

According to Bob Lazar the Area-51 insider, the operative substance in a UFO as far as it's antigravitic capabilities are concerned is a synthetic element we would refer to as element 115, which in the same family column on the Periodic table as Bismuth, and it displays even greater gravitic properties when excited.