The easiest option would be to use a rollable table. I believe this would also work (if my tired math is correct): /roll {1d6-3,0d0}kh1
or
[[{1d6-3,0d0}kh1]] The only issue is there's no easy way to set the number of dice you want to roll this way. You'll have to repeat the pattern for each additional die, like this: Rolling two special dice and adding them together: /roll {{1d6-3,0d0}kh1,{1d6-3,0d0}kh1}
or
[[{{1d6-3,0d0}kh1,{1d6-3,0d0}kh1}]] Rolling two special dice and keeping only the higher roll: /roll {{{1d6-3,0d0}kh1,{1d6-3,0d0}kh1}kh1}
or
[[{{{1d6-3,0d0}kh1,{1d6-3,0d0}kh1}kh1}]]