On this page of the guide to Halo Infinite, you will find the location of the 3rd skull - Mythic. This is one of the skulls you can obtain while playing through the campaign, namely one of the main missions - Command Spire. We've posted detailed instructions on how to get to skull number three.
Location of the 3rd skull - Command Spire main mission
You can only obtain this skull during the Command Spire mission. It's not available during free exploration of the Zeta Halo ring.
During the Command Spire mission keep making your way through the rooms until reaching the one pictured above. You'll notice some containers which move horizontally, not vertically like the ones in previous chambers.
Go to the rear left corner of the large room, to the place where the transported containers turn 90 degrees. You should get rid of all the enemies nearby so they don't interfere.
Wait for a good moment to grapple yourself to the moving container - you must, obviously, use the grappleshot.
Immediately after grappling yourself you must act quickly, as the container will keep on moving. Look up and grapple yourself to the upper shelf. If you can't see the grappleshot's crosshair you should make a long jump.
Repeat this, constantly attracting yourself to higher shelves, although you don't need to rush anymoer. You should eventually find yourself on the same floor as the door pictured above. Go through the door.
The skull is in a secret room behind it.
Mythic skull - how it works
The Mythic skull gives enemies more health making them tougher and able to soak up more ammo and grenades.