3d6        # simplest one
3d6+3      # with a result modifier

3d8r1      # reroll and discard any 1
3d8rlt3    # reroll and discard any lesser than 3
3d8rgt6    # reroll and discard any greater than 6
3d8rgt6+2  # reroll and discard any greater than 6 and add +2 to the final result