They're mostly in the 3D Modelling section. The Halo 4 stuff in particular has its own release thread in the 3D Modelling section - I'd know, I was the one who released and uploaded the files. The Halo Reach stuff is likely in the Sticky section by now, and there's no full Halo 3 Assets thread yet.
Come to think of it, I should change that.
Either way: the assets exist, if you're willing to tickle below the surface and do a little bit of hunting. Without meaning to sound disrespectful, the files aren't too hard to find if you have a little patience and time - I'm hoping that, within the next week or so, I'll have a full Sticky or a dedicated Files section for assets.