CPU3 is currently removed from the scheduler, so not running any user space code (i.e. rapps). It handles hardware interrupts that need low latency, for example from Wi-Fi and Ethernet devices. I've experimented with enabling it in user space but it will most likely need to be reserved to keep the connection as fast, reliable and stable as possible.
Here's a screenshot of my CPU usage while i do a 1Gbit download, with CPU3 active: