Extra roll (for a third hit) in enemy Action?

I have a few enemies that hit 3 times, but there are only 2 built in places for a "hit" with damage calculation. I've tried everything I can think of to make it roll and additional attack, but I can't get it to work. For example, this monster hits for bludgeoning damage, fire damage, and cold damage all in one attack. How can I get that third roll to work automatically like the other two hits do? Please explain in detail if it is possible... I'm really bad with this kind of stuff. lol :)  Thank you guys!!  

If you are wanting the 3d6 to resolve to a number, then you just need to set them up as an inline roll:  [[3d6]].

Kraynic, this is exactly what I was looking for - so simple! thank you!