(Thinking about the far future, here.) Inertial fusion using Deuterum and Helium 3 would solve a lot of these problems. Of course, He3 is rare here on Earth.

I don't think mining the Moon for He3 makes much sense. It's just too rare in the lunar soil.

Instead, my favorite concept is mining it from Uranus (whose gravity at Earth-like pressures is actually slightly less than 1g...). There's vast amounts available at useful concentrations (in addition to deuterium). You'd need a reusable two-stage nuclear thermal rocket to get it back to orbit, but luckily there's lots of hydrogen for propellant in the atmosphere (unlike Earth where that hydrogen must be chemically split from water or methane) and we've built and tested nuclear thermal rockets before (with more recent NTR designs achieving sufficient performance for such a vehicle to close). I'm not a fan of NTR for Earth launch (a lot of cost, plus it actually requires a lot more energy since all the propellant is hydrogen, instead of a mix of methane and mostly oxygen), but it would be enabling for Uranus launch.

Here's a concept for mining Helium 3 in Uranus using floating hot-air balloon mining facilities: https://solarsystem.nasa.gov/docs/5.6_Reinert.pdf