This doesn’t sound quite right to me — are you referring to screenspace AO? Baked AO can give results just as good as GI (at least in terms of the soft shadows) I think, as long as you have a nice UV unwrapping and spend the time baking AO offline.
Once you bake the lightmaps you can export them with the 3d model in GLTF and threejs should automatically load them.
Unfortunately not – glTF files can contained baked AO but not lightmaps. But you could attach the lightmap to the emissive slot in glTF, and then just swap it to .lightMap after loading in three.js easily enough.