"The domain name must not be less than 3 characters", while hp.com is a valid domain name? [closed]

When I try to register a domain name in register.com that is less than 3 characters, it displays this message: "The domain name must not be less than 3 characters"

Why it's not possible to register a domain name less than 3? hp.com is only 2 characters?

How's that possible?

There's no technical reason; as you observe, hp.com is registered, as is x.org. The rule is put in place simply to avoid fights over excessively short domain names, which would be particularly valuable.

As an aside, I believe that all .com names of four characters or less are already registered...

HP.com was registered on March 3, 1986. That was the ninth domain name to be registered.

I don't think they needed to worry about any Johnny-come-lately registrars back then.