Horizon Forbidden West

There are 8 Relic Ruins in all, in Horizon Forbidden West, which are basically remnants of the Old World, ruins which contain mini puzzles in them. Completing them nets you an Ornament, which can be taken to Hidden Ember near Las Vegas, and given to Stemmur, after you’ve done the quest, Nights of Lights (provided you’ve already got Poseidon, that is). Do it, it’s fun! You don’t need to do that for this trophy though, you just need to complete 3 of the 8 Relic Ruins.
The first 3 you can do are as follows:

• The Daunt - south of Chainscrape
• No Man’s Land - east of the Barren Light Salvage Camp
• Restless Weald - north-west of Plainsong

The rest can be found below, if you’re interested:

• The Dry Yearn - east of Scalding Spear
• The Stillsands - east of the Salvage Camp in the Stillsands
• Runner’s Wild - west of Scalding Spear (slightly north too)
• The Long Coast - north-east of Thornmarsh
• Isle of Spires - west of Legacy’s Landfall in San Francisco