Three characters on the .org, and it’s symmetrical. B3B reads the same forwards and backwards, which makes it pleasant in a logo and easy to hold in memory.
The shape also nods at B2B without being it, which is a useful bit of distance. A business-facing organisation gets the association without the generic quality of the actual acronym, and there’s no confusion with the thousands of things already using B2B.
With no fixed meaning, it’s open to whatever the buyer builds: an association whose initials fit, a business network, a technical body, or a foundation wanting a compact identity.
The .org gives it institutional weight that the same string wouldn’t carry elsewhere. Three characters plus .org is close to the default shape for an established body’s web address, and the ordinary combinations have been gone for decades.
Symmetrical, short, permanently scarce. Offers welcome.