Why isn’t my Beats Solo 3 headphones supported for the custom audio settings

Greetings slim245,

Unsure of what exact settings you're looking for. However we have some information for you to look over. Check out the "Headphone Accommodations" section from this page to learn more: Accessibility - Hearing - Apple. It looks like at this time, you can customize headphone audio levels on your iPhone for the following device types:

Headphone Accommodations works with these headphones:

AirPods Pro (1st and 2nd generation)

AirPods (2nd and 3rd generation)

AirPods Max

Apple EarPods (with 3.5 mm Headphone Plug or Lightning Connector)

Powerbeats

Powerbeats Pro

Beats Solo Pro

Beats Fit Pro

You can learn this information and more here: Customize headphone audio levels on your iPhone or iPad - Apple Support.

For any suggestions you may have to improve Apple's products or services, you can reach out to Apple directly via their Product Feedback - Apple page.

Kind regards!