I see a lot of people on this forum rendering C-3PO as "C3-P0", moving the "-" and adding a zero in place of the "O". Is this an error where people are just shifting C-3PO's designation to more closely match the astromechs, or is this some forum in-joke that I'm missing?

I think it's just a common typo, or people being unaware or confused.

Like Mikael Hasselstein said, i think it is just a common typo.

And it's not just here. It's everywhere.

Guilty

Well, OK then. I'll consider this my PSA.

I blame my phone did auto correct

I blame my phone did auto correct.

Don't look at the Regionals thread too closely.... my summaries are full of C3-P0s! (I will eventually edit it to be correct...)

Now I'm wondering how I've been typing it

It's See (pause) Three Pee Oh

In the original trilogy, he is called/referred to as "3PO" by Luke on several occasions. The "C" is eliminated, only making sense that the "-" goes after the "C."

More likely to be a languages thing. On spanish it is C3PO, since we do not say zeros that way, so that's how we call it. Also, What Patox and GrumpyMuffin are saying, i had always heard it as C-3PO, but i would like to know the correct one (except the O or 0, since that's just pointless except when i am writing in this forum ;P)

It's just a somewhat vapid typo.

I also was under the impression that it was a zero, not a letter on the end. Just checked the credits of a new hope though and it does look more like the letter.

It's the letter O

I am C-3PO, human-cyborg relations.

Check out the Wookieepedia entry, copy-paste it into a word processor, and compare the O [letter] to the 0 [number].