Why is the title 'Three Kings'

To quote Conrad Vig: "We three kings be stealin' the gold...".
The title was taken from a line in his song.

What? you ever heard of the 4 musketeers? No? but there are 4 of them strange.....

a reference to the 3 kings in the bible that visit jesus?

"we be stealin the gold"

as in murr, frankincense, Myrrh, gold!

Yeah, the title really doesn't make sense. I mean, obviously the Three Kings would refer to Clooney, Wahlberg, and Ice Cube, yet it's Conrad who sings the song. He's not even one of the "Three Kings."

Now, if it had been Wahlberg or Ice Cube singing that, then it would have made perfect sense. Sorry Conrad, but you're on the outside looking in.

I believe it's a reference to "We Three Kings of Orient Are," also known as "The Quest of the Magi," a Christmas carol composed in the mid-nineteenth century.

We three kings of Orient are
Bearing gifts we traverse afar
Field and fountain, moor and mountain
Following yonder star ...

In the story of the birth of Christ, three kings from the far east follow a star, bringing gifts of gold, frankincense and myrrh to the newly-born Messiah. Spike Jonze is spoofing the song when he sings "We three kings be stealin' the gold ..."

[SPOILERS]

Why call it "Three Kings" instead of "Four Kings"? Not only is it playing on what the filmmakers probably assumed was a common cultural reference, but it also foreshadows that only three of the four who set off in search of the gold make it home alive.