3 custom domains - one is default - another coming up with double ‘www’

When you set a default, any other domains you have defined in your webflow site hosting settings automatically get redirected. You do not need to setup any redirects.

For domains that aren’t defined in Webflow’s hosting settings, you do need to setup your own redirects, but it sounds as though you’ve done that fine.

The www.www is often a DNS caching issue.
Does the site work fine if you access it from your phone or another machine?

You can try this tool as well-
https://support.webflow.com/tools/domain-checker