How to adjust Apple TV volume?

You would need to separately adjust the volume using the remote for your TV or speaker system when using that model. The 3rd gen is also quite old at this point, around 10 years, and has limited support from Apple (not to mention channels from other providers increasingly dropping their support). May be worth upgrading.

… on appletv remote 3rd generation

There is an Apple TV box (3rd generation), but no model remote is named ‘3rd generation’.

Identify your Apple TV remote - Apple Support

Please use the same naming convention as Apple uses.