When you are supposed to roll the dice and keep several of the highest, it is notated like this: 6d6h3 (roll six d6 and keep the highest three). When you are supposed to roll the dice and keep several of the lowest, the notation is as so: 6d6L3 (roll six d6 and keep the lowest three).
When a roll calls for advantage on a roll, roll the dice twice and keep the highest result. When a roll calls for disadvantage, roll the dice twice and keep the lowest result.