The installer will ask for "Setup Disk 2" and "Disk 3." In VirtualBox, you must manually "insert" the next floppy image via the devices menu while the VM is running.
Microsoft chose the version number "3.1" to match the consumer Windows of the day, suggesting parity. But internally, NT was a titan. It was built on a microkernel architecture, featured the Win32 API (which would power Windows 95 and beyond), and was designed to run on multiple architectures: Intel x86, MIPS, and DEC Alpha. windows nt 3.1 iso
However, there are a few options to obtain the Windows NT 3.1 ISO: The installer will ask for "Setup Disk 2" and "Disk 3