DnD 5e Monster/NPC Stat Block Attack (Damage Types)

Is there a way to add a 3rd damage type to an attack right now? I mean something like: on a hit this does 1d8+5 piercing damage, 2d6 poison damage and 2d6 necrotic damage.

There are no attributes for it, but you could add it to the description field of the attack, as: Plus [[2d6]] necrotic damage

If you're willing to use custom macros, the npcdmg template neatly stacks with itself: &{template:npcdmg}{{damage=1}}{{dmg1flag=1}}{{dmg1=[[2d6]]}}{{dmg1type=Piercing}} &{template:npcdmg}{{damage=1}}{{dmg1flag=1}}{{dmg1=[[2d4]]}}{{dmg1type=Fire}} &{template:npcdmg}{{damage=1}}{{dmg1flag=1}}{{dmg1=[[1d6]]}}{{dmg1type=Lightning}} &{template:npcdmg}{{damage=1}}{{dmg1flag=1}}{{dmg1=[[1d4]]}}{{dmg1type=Force}} Keith's version is much easier and can be done straight through the sheet - bear in mind it won't automatically roll crit if it's not in the 2 pre-made damage sections. Another options is to stack two of them into one damage field,  [[2d6]][Bludgeoning]+[[2d4]][Fire] , and mouse-over to see the breakdown.

I had a creature in a recent adventure that also did 1d6 fire and 1d6 necrotic when it hit with a longsword attack. I put 2d6 in damage type two and "fire and necrotic" in the description.