PPE is a jack of all trades, master of none type degree, if you'll forgive the expression. It's very broad. Only the very good people will be able to successfully get a good job in any one of these three fields because you'll have covered so much less than just a single honours student.
For example, if you wanted to become an economist and so basically have to do a MSc, I'm sure you'll get in but it'll be tough catching up having covered 1/3 of what your peers will have. This is why only the good people who do it can do really well in a specialised field. These people certainly exist and I've met many of them in the economics industry for example but all did PPE at Oxford, think I've only met one non-oxford person who did PPE and was a professional economist (went to Warwick).
Bear in mind that this is just my experience so I doubt will be fully representative, it's not like I can know everyone haha. I'm sure there are lots of very successful PPE students, it's just in my experience over various economic and banking internships and a grad role, I've basically never met a non-oxford PPE economist at a top government department/bank/Financial institution/consultancy.