The most striking difference would be that of size. The helium-3 atom is much smaller than a uranium atom, having only 2 protons, 1 neutron, and 2 electrons in the outer energy level. Uranium, by contrast, is much larger, having 92 protons, 143 to 146 neutrons, and 92 electrons arranged in various energy level structures. The helium-3 atom is much less dense and therefore lighter than the standard uranium atom. Uranium is much more dense and is heavier than helium-3.
Uranium is one of the standard fuels used in nuclear reactors. It breaks down by the emission of an alpha particle, which bears a striking resemblance to helium-3 in that it has 2 protons and 2 neutrons, but no electrons, giving the alpha particle a 2+ charge. This emission of alpha particles gives off energy in the form of heat energy, which is used to turn water in the nuclear reactor to steam. The steam, in turn, is used to drive the turbines of a dynamo to generate electrical energy.