I would call the parents and let them know at least. Up to you if you call to pick up as only you can decide if it's getting too much to deal with. Three is quite a lot.
My own son used to get horrible nose bleeds when he was younger. They could literally pour for 15-20 mins and for no reason in terms of not banging his nose. He's grown out of them now thankfully but they lasted most of his Elementary school years and seemed to be triggered by emotions. He is quite a sensitive child so any conflict amongst his peers, any time he was made centre of attention which he's not comfortable with (school awards and being called up in assembly), he'd get stressed out and the nose bleeds would begin.