Halo 3 Spartan Laser polycount is 2511. If they start doing what some other game studios do with their first person weapon models(only model the parts that you'll actually see in first person), then they're saving quit a bit, and can put more detail into what you see. All the extra little details in the Reach Splaser is probably only an additional 500-1000 polys. Really not that much of a difference.
And on top of that, IF Campaign and Multiplayer are on separate discs, they have far more space for larger textures, and mesh data.
So, Halo 3 Engine could handle the models, yes, but it wouldn't be practical, especially considering those same models would have to be used in Halo 3 multiplayer.