Everything I find on Google just tells me where to find them, but I have the map, so I don't need that. I just don't know what I do with them once I have the sets. It says you trade them to special merchants in Meridian, but the 4 merchants there don't have any options for trade for them. Do I need all 3 sets to have them offer stuff, or is there another merchant I don't know about? Thanks

Set 1

This is found in the Ruins where you fall in as a kid.

Metal Flower - Mark 1 (A)

Head there as an adult, and go all the way back to where Aloy first picked up the holographic display piece. There is a nearby doorway that is blocked off by stalagmites. Smash through them and inside you will find this Metal Flower.

The second flower is located in the bottom-left of the map, in the first area of the game.

Metal Flower - Mark 1 (B)

The flower is up the mountain a bit, requiring some climbing and rope sliding to reach. Start at the house of Rost and Aloy, walk south-west in the direction of the flower until you get to the ropes.

Found in the second area of the game, just a bit North of Main Embrace Gate.