There is a rift in Lake Hylia that has a glowing spot where Tri can create an entrance.

Find Tri's Friends, Part 1

Go west to where there are some dark Tanglers in some sideways water. Use monster echoes or swordfighter form to defeat the Tanglers, then go down to the bottom and check on the glowing object to rescue one of Tri's friends.

Find Tri's Friends, Part 2

Go to the surface and go north. When the path splits, go northwest. You reach a large body of water bordered by trees. Go to the northwest corner of the area, then dive down and go through the west wall. Bind to the glowing boulder and push it out of the way, and check on the glowing object to rescue another of Tri's friends.

Find Tri's Friends, Part 3

Go east through the underwater archway, then go to the surface and go southeast. Jump from your current body of water to the one to the east. Then go south until you can go east to a new area.

Go to the right side, where the water goes up higher, and go to the surface there, but watch out for the dark Spear Moblin to the east. Use water block echoes or other echoes to make your way to the highest area of water to the east. Then bind to the boulder and pull it downward and to the side so you can reach the last glowing object. Check on it to rescue the last of Tri's friends.

There will be a cutscene. Tri's level gauge will increase. You receive 2 Might Crystals.