Talk Elections

Does the constitution mention such a scenario? 

As far as the Constitution is concerned, no one votes for President on Election Day, rather the State Legislatures have chosen to allow them to vote for Electors. If for some strange reason, all 538 Electors nominate different people, then it would be a wide open race in the Congress with the House having 538 choices for President and the Senate having 538 choices for Vice President.

If it's a simple case of nobody running then people could still write in candidates. We'd probably end up with a celebrity or something. :P

Isn't it limited to only the top three?

Technically they're all tied for first if every elector nominated a different person, so that rule isn't relevant.

Yes... but if 538 people tie for first slot...

Of course, in theory electors do not have to cast a valid vote (also, what are state procedures if noone wants to be an elector?) so it could still theoretically happen.

And no, the Constitution does not address such fancy issues. Very little of actual US constitutional law and custom is actually Constitutional text.

The 12th amendment does not account for a tie any diffrently, so the house would have to pick 3 to decide from.

That is absurd. The House would pick three for itself to decide from - so a two-round vote between all 538? Where do you believe to be drawing that from, exactly?

In California, write-in votes are discarded unless a candidate registers as a write-in candidate.  Is it like that in other states?  Because if so, no write-in votes would be counted.

South Carolina does not allow for write-in votes for Presidential Electors.

12th Amendment says: "if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President."

The House would only be abel to choice from 3.  How the 3 would be picked is left unsaid and, if there was a tie involving more than 3, deciding that would probably cause a crisis in Congress.

The most straightforward reading of the clause you quote is that, if four or more persons are tied at the top, then nobody is eligible to be chosen by the House.
Which is... interesting.

The "highest numbers" language resurfaces in the VP tiebreaking procedure, though... and there it seems likely that the intended meaning is indeed "all the people with the two highest occurring [non-0] totals"  as opposed to "the two people with the highest totals".

The House language is slightly different in including that "not exceeding" though.

Huh, yeah. Based on the exact wording of the Twelfth Amendment it looks like the House of Representatives wouldn't be able to vote at all if there were a four-way tie in the Electoral College.

I'm not sure what would happen in the event of a four-way tie - a special election, another term for the sitting President, or Speaker of the House becomes President?

I disagree.  If there is a four way tie, then "from the persons having the highest numbers"  there can still be three to pick from who have that highest number, with the fourth left out.

What if the Senate were still able to pick the Vice President? Do they go under different rules?

Somewhat, yes.

If the House hasn't settled on a President yet by the time term starts, the VP-elect serves as Acting President. That question is settled IIRC.

Also if there is no President-elect or Vice President-elect chosen by the start of the term, the Speaker serves as Acting President, if eligible.

A highly unlikely scenario. There's too much money at stake and too many corrupt politicians around who are willing to play that game.

Mickey Mouse!