Original Equipment Manufacturer. Legitimately, OEM licenses are pre-installed on new PCs and tied to the motherboard. In pirated ISOs, “OEM” is often misused to imply pre-activation via fake slic tables or bootloaders.
Note: In some releases, “3in1” means Home + Pro + Pro for Workstations, but given you specified “Pro 3in1,” it refers to three variants of Pro. Windows 10 X64 22H2 Pro 3in1 OEM ESD sv-SE AUG ...