It says I've got about 3 GB and it won't let me download something for 2.2 GB. Why is it saying this and how do I successfully download it?
The default configuration would download to the same APFS container that macOS is install to. It would not be wise to fill the container to a point where there would so little free space.
You can change the Safari preferences to ask where to download. You will need to select a location that has enough free space. Or, create more free space for the volume where Safari is currently downloading.
See the Apple website How to free up storage space on your Mac.