Handling "rel hreflang" for specific languages in countries "en-US" and generic languages ex: "en"

Question

If I have 3 english versions of my website:

/        FOR USA
/au      FOR AUSTRALIA
/gb      FOR United Kingdom

I would add them to my Sitemap using rel hreflang to serve those 3 countries:

Sitemap.xml

<url>
  <loc>https://example.com</loc>
  <xhtml:link href="https://example.com"    hreflang="en-US" rel="alternate"/>
  <xhtml:link href="https://example.com/au" hreflang="en-AU" rel="alternate"/>
  <xhtml:link href="https://example.com/gb" hreflang="en-GB" rel="alternate"/>
</url>

QUESTION

What if I want all english speakers that are not in one of those 3 countries to be led to my en-US page, what should I do?

OPTION #1

Keep it just like this and let Google decide which is the best version to serve to a english speaking user that is not USA, AU and GB.

<url>
  <loc>https://example.com</loc>
  <xhtml:link href="https://example.com"    hreflang="en-US" rel="alternate"/>
  <xhtml:link href="https://example.com/au" hreflang="en-AU" rel="alternate"/>
  <xhtml:link href="https://example.com/gb" hreflang="en-GB" rel="alternate"/>
</url>

OPTION #2

Omit the en-US and change it to just en so it can get the traffic from USA and also from other countries that speak english other than Australia and UK ?

<url>
  <loc>https://example.com</loc>
  <xhtml:link href="https://example.com"    hreflang="en" rel="alternate"/>
  <xhtml:link href="https://example.com/au" hreflang="en-AU" rel="alternate"/>
  <xhtml:link href="https://example.com/gb" hreflang="en-GB" rel="alternate"/>
</url>

OPTION #3

Keep the en-US and add a duplicate hreflang to en.

<url>
  <loc>https://example.com</loc>
  <xhtml:link href="https://example.com"    hreflang="en" rel="alternate"/>
  <xhtml:link href="https://example.com"    hreflang="en-US" rel="alternate"/>
  <xhtml:link href="https://example.com/au" hreflang="en-AU" rel="alternate"/>
  <xhtml:link href="https://example.com/gb" hreflang="en-GB" rel="alternate"/>
</url>

Answer 1

Option #3: use a line for en, in addition to the three locale-specific lines. A Google representative has confirmed that multiple targets for one URL is allowed.

You do want to target traffic to the appropriate locale-specific site, where one exists. That rules out option #2.

Option #1 would work, up to a point, in that the search engine will still serve your pages for searches outside the explicitly served locales. The snag is that you wouldn't be able to control which page is being served to them.