A lot of people will interchange an "o" for a zero when speaking them aloud, like with phone numbers and postal codes, so I'm thinking this is the same here, but with nicknames like "threepio", I'm not so sure.
I'm looking for the canon answer here.
A lot of people will interchange an "o" for a zero when speaking them aloud, like with phone numbers and postal codes, so I'm thinking this is the same here, but with nicknames like "threepio", I'm not so sure.
I'm looking for the canon answer here.
The script spells it with an 'O':
An explosion rocks the ship as two robots, Artoo-Detoo (R2-D2) and See-Threepio (C-3PO) struggle to make their way through the shaking, bouncing passageway. Both robots are old and battered. Artoo is a short, claw-armed tripod. His face is a mass of computer lights surrounding a radar eye. Threepio, on the other hand, is a tall, slender robot of human proportions. He has a gleaming bronze-like metallic surface of an Art Deco design.
Although I can't find his designation used anywhere in the text of the novelization, the 1979 paperback has an inset featuring stills from the movie and short character bios; Threepio's designation appears there as "C-3PO" (with an 'O', not a '0'):

It's faded, but you can clearly see an 'O'.
StarWars.com, the scripts, and the novelization all explicitly say it is C-3PO.
To confirm this, I copy-pasted the text from the StarWars.com page shown below and then typed it out with a zero instead in a font that distinguishes the two characters. You can see the difference in fonts.
"C-3PO" – directly from starwars.com
"C-3P0" – typed with a zero-character

Similarly to this, I checked the novelization and the scripts. Searching for "C-3PO" returns numerous results. Searching for "C-3P0" returns none.