I don't necessarily need a tutorial (though it would help) but I'm wondering how to make 3d explosions? Should I use a particle explosion or sprites of some kind or shaders or something else? Never done explosions in any language so have no idea where to start and just a tiny inkling of the possible methods.

What methods would you suggest I research?

Usually you would use a particle system, but you can have animated sprites for the images (you can render these in Blender or draw them).

There are many ways to do this, but lowest CPU demand would probably be to just spawn a 3D sprite and use a spritesheet texture of a prerendered explosion. I haven't used the 3D sprite node in godot myself but if it can't use a spritesheet normally it probably can use it with a custom shader.

Another suggestion is to use shaders (run by GPU so CPU usage = close to 0%).

It's worth pointing out as well that because particle systems across most game engines are so customisable down to each individual particle it's mostly down to your setup whether or not they're resource hungry or not. Don't for example do something like spam 10,000 particles to get that exact effect you want especially if it's duplicated multiple times at once, if you're sensible you'll be fine and the others have already given some good methods to follow.

You can look for tutorials on particles and find resources, but most of them are for unity. Although they are not exzact, studying unity particles could help you make them in Godot. It's about getting the basic ideas of it.

Well I delved into particles. I was hesitant at first because the examples I saw earlier (when I wasn't actually trying to learn yet) used 3D shapes and looked like quite a load. I found a couple of fire examples that use bitmaps and billboard mode that actually look quite decent so I'm experimenting with that for the moment. I have to see if I can get some decent looking booms and small fires and smoke.