I had one run where I received bonus shards for all 3, so it appears I can confirm it’s not an either/or situation.
Also, where did they state the 30% drop rate? I know that’s what they’ve stated for normal hard node drops, but I never saw them state that for bonus shards.
If I had to guess I would bet the bonus shard drop was something more like 10% per character, for a total of 30% bonus drop chance.
Under that assumption, your first example of zero drops on 8 runs (.7^8) 0.058 or 5.8% or 1 in ~17 attempts. That certainly seems reasonable to me, and anecdotally seems to match what I experienced.