There’s one 3.3v regulator that’s always powered on the Mayfly (anytime the Mayfly power switch is ON, even if the processor is put to sleep), and that can be found along the top (inward end) of each of the 20-pin headers, helpfully next to the GND pin you’d also need for sending power to an external device. The three switched power options (3.3v_SW, 5v_SW, and 12v_SW) are available on the other end of the headers, and are controlled by pin D22. I’ve attached a cropped snippet from the Mayfly schematic showing the header pinouts, since it’s sometimes hard to read the labels on the actual boards. This attached pinout image applies only to the v1.0 and v1.1 boards. Previous versions had different pinouts because of the different design changes between versions. All the full schematics are available in the Board Schematic section of the Mayfly hardware page.