What's correct "A, B and one other alphabet have been updated" or "A, B and one other alphabets have been updated"
Based on your comment, I think you intended your sentence to be:
A, B and one other letter have been updated.
Here, "one other letter" is considered a single noun phrase. It is separate from A and B. Therefore, your sentence is correct.
One way to adjust the sentence is to add an extra comma, called the "serial comma". In my opinion, this makes the distinction a bit more obvious:
A, B, and one other letter have been updated.
(Note that use of the serial comma — or "Oxford comma" as it is colloquially known — is a matter of personal preference. I just happen to like it.)