manufacturer like airbus do not even consider possible that one of his planes could suffer by a triple Hyd fail
Yes they do.
They do the maths and they come out with chances of it happenning so low, that it doesn't justify the installing of the fourth system.
Send PM to PBL, if you'd like to see actual numbers and methods of getting them.
However, PTU that doesn't shut itself off with HYD LO LVL is something that Airbus could have done bettter and I hope it will be rectified.