To put it shortly; triskelion key is a key that requires 3 triskelion key pieces to assemble. (
http://runescape.wikia.com/wiki/Crystal_triskelion)
These pieces would be dropped in order to prevent you from getting multiple duplicate key pieces in a row. Chest where to use these could be located deep in the wilderness. Inside Rogue's Castle is my suggestion.
These key pieces would be dropped by every monster that drops either hard or elite clue scrolls so it would be a tertiary drop like them.
Drop rate would be 1 in 50 from NPC that drop elite clues and 1/100 from a monster that drop hard clues.
Possible rewards would contain coins, high level uncuts (diamond, dragonstone), dye shards, soft clay, high level hides (red, black), high level bars (mith, addy, rune), fury sharks.
Having 1 or 2 cosmetic items as a rare reward from them could be a possibility. Suggestions what this item(s) could be? Flippers pls