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.
Less than 10^-9 per flight hour. Just like other catastrophic failures.
Would be interesting what the "maths" say about DUAL HYD SYS LOSS.....
Off the top of my head I can't think of a G and Y hyd failure that wasn't caused by inadvertent use of the PTU. Certainly it's the biggest cause by far. It's interesting that despite all the cries from pilots about over automation in FBW Airbuses, pilot control of the PTU is a problem. Should Airbus remove the PTU p/b and automate the system further to prevent overheating and a resulting dual hyd loss?
757 and 767 also have three hydraulic systems, albeit without a PTU.