Hello Sangarunya!
For a damage of 3d8 for a medium creature, it follows the progression 2d6 for a tiny creature, 2d8 for a small creature, 4d8 for a large creature, 6d8 for a huge creature, 8d8 for gargantuan and 12d8 for colossal.
I've just updated the in-game chart with that new column for version 1.40.
I've also expanded the chart with the damage for a diminutive creature (one step below tiny), for colossal+ (one step above colossal) and colossal++ (two steps above colossal).
Thank you! Best regards
