With the extra 100% XP buff… does that make the sleeping bag an extra 6%? For a 106% total? Or is it simply a 103% bonus combined?
Thrown for a loop on this one 
Generally blizzard has done addition not multiplication for these kind of things.
So if you have rested 100% mob exp, and you have 100% from this buff, you’d be getting 200% exp and not 300% exp.
So I’d imagine the sleeping bag would give 100+3% bonus exp
bonuses are calculated seperatly so lets say a mob give 100xp. it would be an extra 100 xp + 3xp for a total of 203
Okay that makes sense ty so much 
Ok, but what if youre rested, with the buff, and the sleeping bag, BUT its also a full moon.