Helium-3 is a fuel for fusion that does have its benefits, but most scientists are focused on Deuterium-Tritium reactions. On the moon, the most concentrated regolith has only 50 parts of helium-3 per billion. That is super sparse. Like, we may as well be sifting uranium out of Earth's oceans at that point. And if we're doing giant space-based megaprojects, the gas giants probably have higher concentrations of helium-3 or we could just beam power around from giant solar panels.