The spiritual meaning is derived by looking for the Christological elements with puns and riddles.
Jehosephat - God - judge
Decision - judgement
Shittim - wooden branches from the acacia tree, a pun of Sattah - turn aside, from a root meaning pierce
The scenario that is portrayed in Joel 3 is that all the nations have been gathered for the final battle against God/and they receive judgement. But the judgement is taken by Christ on the cross.
As a result of the cross, their children are sold to Sabbeans (drunkards) as a riddle of the Holy Spirit at Pentecost.
Instruments for tilling the soil, or plowing the rough soil are turned into swords/(the Word of God), and we are made strong in the Lord.
The tribulation of Christ was great, and the sun and moon turned dark upon the cross.
The new wine is the grace of removing sin, not merely the old wine of being long-suffering over it.
These are just the highlights of the hidden picture of Christ in Joel 3.