To make the Trix Amulet you need to select the upgrade option on one of the Nax Amulet, Yix Amulet or Tam Amulet while having all three in your inventory. After upgrading, the Trix Amulet becomes untradeable and you will be unable to revert the upgrade, so make sure you're ready to give up the cost of the amulets before making the upgrade.
The Trix Amulet can later be upgraded to a Trix Amulet (i) using a Trix Orb. The Trix Orb is an untradeable item only obtained by donating or as a drop from Necromancers. The Trix Amulet (i) has all the same stats as the Trix Amulet and it also banks your items!