If you want to get the boot open then there should be access from the rear side (in the boot) of the door
GoogleaIs "audi a3 emergency boot release" for lots of indications and the release method.
If you want to fix it then it's probably a solenoid so again Google something like "audi a3 boot latch fault" that should give you several videos on how to strip the door and investigate the problem.
If your going too check the pcv just take the oil filler cap off see if that make a difference she may be on its way out normally you get a whistling sound after a while, if no difference I'll be more looking at coil plugs and the wireing spark plugs as the car starts fine but once warm it goes lumpy