In the US, if a majority voted for a third party for the presidential election, would he be elected?

Yes and no.  First, the way the US presidential election process works, majorities don't really count.  In the last 20 years, two Presidents 'won' with the losing candidate getting millions of more votes (GW Bush, Trump).  So an outright majority is not a sure path to victory (but usually is).  But any third party candidate that wins enough votes in enough states to win an electoral college majority can win. 

The electoral college would have to vote him in.

POTUS is not elected by a majority, they are elected by electoral votes, with each state setting their own policy about how electors are expected to represent the will of their state.

If a 3rd party candidate can win enough states to get the most electoral votes then they would become POTUS.

Sure. But it ain't gonna happen.