Keep in mind - I don't think there's a real standard for "Bootloader stages" so a system designer (or anyone) can call any step their platform takes to initialize anything they want without consequences.
Firmware (i.e. the BIOS) chooses a device to boot from and executes the boot code stored in the boot sector (MBR) of that device
This is the way traditional BIOS boots anything - traditional BIOS has no knowledge of file systems, so it can't load a file directly. So BIOS will ask the device for LBA 0, put it in RAM at 7C00:0000, and point the CPU there.
That is stage 1 - it's primitive, but it's a step nonetheless - so that's stage 1 (1BL).
LBAs are 512 bytes, and the last 64 bytes of LBA 0 are the partition table and boot signature, and that's not enough code to do very much, so usually there code that loads a bootloader with more capability. That would be your 2BL.
Of course, maybe you can fit enough code in 512 - 64 bytes to load your OS. I believe DOS was able to load the first part of itself through this method (DOS was in a couple files on a disk - IO.SYS and MSDOS.SYS - and I believe the 1BL will get IO.SYS into memory which then loads MSDOS.SYS, but IO.SYS is used by MSDOS.SYS for more than just 2BL.)
The MBR code usually chooses a partition to boot from and executes the code stored in that partition's boot block (VBR)
So a VBR is just MBR code that's not at LBA 0 of the disk, but the first LBA of a partition. I don't see the utility of splitting that into 2BL and 3BL, but someone could consider those separate steps if they really wanted to.
Any talk about more than 1BL/2BL that I've seen is usually to do with decrypting the next stage and verifying that it hasn't been tampered with, on platforms such as Microsoft's original XBox or other game consoles.
Windows Bitlocker could be said to add additional steps - but technically the Windows kernel is already running during that process - that's how you see the Bitlocker screens in the case your recovery key needs to be entered.