[Thorondor]

Since a fusor normally runs on deuterium fuel, wouldn't the output be helium-3 with a mass of 3 grams/mole?

Not that it makes a big difference - producing helium with a fusor remains extremely impractical.

[GeorgeKangas]

Oops! I used He's atomic number (2) instead of its atomic mass (usually 4).

I believe deuterium fusion events mostly produce He4, occasionally He3 + neutron. So we'd actually produce between 3 and 4 grams.

[Thorondor]

No, deuterium-deuterium fusion reactions usually produce helium-3 and a neutron. Due to the conservation of energy, producing helium-4 requires the emission of a gamma ray. This happens rarely because, since the strong nuclear force is stronger than the electromagnetic force at small distances, fusion reactions tend to release energy as protons and neutrons rather than gamma rays.