Confession time - I have a maths degree.
It's been ten years since I did any statistics, but still I am very embarrassed about my RNG rant in an earlier post. Mainly because the probabilities are complete bollocks.
So let's try again. Ten man bosses drop two items chosen from a list of ten. Suppose you want a particular item to drop - the chance of that item being the first epic that drops is 1 in 10. The chance that the item is the second one that drops is 1 in 9 (because the same item can't drop twice). Thus the chance of not getting that item 9/10 * 8/9 = 80%. So you have a 20% chance of the item dropping.
Where the boss drops two items, the probability of neither appearing is 8/10 * 7/9 = 62%. Your chance of seeing either or both of them is therefore 38%.
Finally, for three items, the probability of seeing at least one of the three is therefore 1 - [7/10 * 6/9] = 53%. I apologise to all three of my readers for mucking up the maths.
But then of course if the RNG hates you then unless you have some good friends with 25 man BoE gear to give away, then you have no hope. I still see the posts of the people who had to wait weeks and weeks for their Red Sword of Courage from UP heroic and feel grateful it only took me 7 or 8 tries.