To be exact, the requirement said: If you worked for only one employer, you need provide three professional references. if you worked for more than one employers, you need to provide two references from each employers. In all references from each employer, one must be from your direct supervisor, and another from a professional engineering which directly supervised your engineering design work, another can be from your engineering work. Well, that's how they require after you passed the PPE exams. It's a little bit different than what's said in the documents.