Where, in Israel, is the valley of Jehoshaphat?

Joel 3:2 I will also gather all nations, and will bring them down into the valley of Jehoshaphat, and will plead with them there for my people and for my heritage Israel, whom they have scattered among the nations, and parted my land.

  12 Let the heathen be wakened, and come up to the valley of Jehoshaphat: for there will I sit to judge all the heathen round about.

I've read some saying that is the same as Jezreel, some say it is from Jerusalem to Megiddo,

and some say that it will be the valley when the mount of Olives is split.

Obviously these are not all the same.

Which one is it?

Or is it yet another valley, not mentioned above?