3u USB-C STM32G4 microcontroller module. Intended for use with keyboards. It is named for its module connector spacing, which is 57.15mm/2.25in or 3 "standard" key spacings.
3u was designed in Altium Designer 19. If you would like to use these design files with free software, you can use Altium's CircuitMaker or attempt to import the project into KiCAD using https://github.com/thesourcerer8/altium2kicad.