In fantasy grounds unity if you type /die 3d6 (roll three dice) k1 (keep 1 highest) It is documented in
https://fantasygroundsunity.atlassia...0/Rolling+Dice
This feature was advertised in one of the kickstarter videos which was a big draw for me.
Here is adding modifiers to the roll:
https://fantasygroundsunity.atlassia...our+Dice+Rolls
At the time of writing this message writing /mod +1 longsword and after that rolling dice with a command /die 3d6 does not add that modifier to the roll. This however works if you grab 3d die and roll it normally. This however works in fantasy grounds classic either way.