Alaina, 16, and McCreery, 17, will go head to head for the 10th American Idol title.
A record 95 million votes were recorded after the semi-final show.
Reinhart, 20, kept her cool, belting out Bennie And The Jets without shedding a tear.
At the end of the song she said: “You’ll be seeing me, this ain’t the end of this, this ain’t the end of this thing – see you later!”
She then embraced her mother and father in the audience.
There were, however, plenty of tears earlier in the evening as each of the three finalists were filmed visiting their hometowns.
The show also featured a sizzling live performance by Nicole Scherzinger and 50 Cent, and the US debut of Italian boyband and tenor trio Il Volo.