Amulet of Double or Nothing

This amulet appears to be glass sectioned off in three parts. Each part displays a different symbol which changes occasionally. Three times per long rest when you make a d20 test, after you see the result but before you know the outcome you can say the command word, “Double or nothing”.  

Roll 3d8. Depending on the number of matches you get, different things happen.

0 matches: you take an amount of damage equal to the 2 highest numbers rolled and treat your d20 roll as if you had rolled a 1.

1 match: You may treat your d20 roll as if you had rolled a 10. 

All match: treat the roll as if you rolled a 20.