For example, you might know that to roll an attack roll you need to roll a "D20 plus your attack modifier".
In Roll20, you would just type /roll 1d20+5.
3Di rolled 1d20+5 and got a total of 16
If you hit and you need to roll 3d6+2 damage, you would just type /roll 3d6+2
3Di rolled 3d6+2 and got a total of 11