who was the king who was surrounded by 3 nations of soldiers, called for a fast?

Question

this kings army singers singing unto god as they went to the batle.

Answer 1

It was Jehoshaphat, the King of Judah. The story you mention is told in 2 Chronicles 20. Here are some key verses and summary

PROBLEM

1 After this, the Moabites and Ammonites with some of the Meunites came to make war on Jehoshaphat.
2 Some men came and told Jehoshaphat, "A vast army is coming against you from Edom, from the other side of the Sea. It is already in Hazazon Tamar" (that is, En Gedi).


RESPONSE: fast & pray

3 Alarmed, Jehoshaphat resolved to inquire of the LORD, and he proclaimed a fast for all Judah. 4 The people of Judah came together to seek help from the LORD; indeed, they came from every town in Judah to seek him.

When the people gathered at the temple, Jehoshaphat prayed for God to keep his promise (to Solomon, recorded in 2 Chr 7) and rescue them if they cried out to him when in distress.

ANSWER:
God's answer came through the prophecy of a Levite singer, with the following 'battle instructions':

15 He said: "Listen, King Jehoshaphat and all who live in Judah and Jerusalem! This is what the LORD says to you: 'Do not be afraid or discouraged because of this vast army. For the battle is not yours, but God's. 16 Tomorrow march down against them. They will be climbing up by the Pass of Ziz, and you will find them at the end of the gorge in the Desert of Jeruel. 17 You will not have to fight this battle. Take up your positions; stand firm and see the deliverance the LORD will give you, O Judah and Jerusalem. Do not be afraid; do not be discouraged. Go out to face them tomorrow, and the LORD will be with you.' "

OBEDIENCE:
Jehoshaphat and the people followed these instructions-- trusting the promise, the army marched out to battle. The text then adds this little detail --

21 After consulting the people, Jehoshaphat appointed men to sing to the LORD and to praise him for the splendor of his holiness as they went out at the head of the army, saying:
"Give thanks to the LORD,
for his love endures forever."

[This was the common worship refrain of the Levites]

RESULTS (God kept his promise):

22 As they began to sing and praise, the LORD set ambushes against the men of Ammon and Moab and Mount Seir who were invading Judah, and they were defeated. . . .

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A couple of notes

a) note that the singing itself was NOT a specific part of the instructions, not the "strategy" God told them to follow. Rather, it was the way in which Jehoshaphat decided to EXPRESS their FAITH in the promise -- basically singing a song of victory BEFORE the battle!

b) again, on the "strategy" -- the point of the passage is not (as some modern-day worship leaders misread it as saying) that they "fought" by their worship-songs. The FIRST step in their strategy was (as always in 2 Chronicles) to "humble themselves and pray" for GOD to act to deliver them. The other step was, following God's instructions, to march out in faith that HE would fight the battle, as he had said. (Again, the singing is not presented as a 'means of fighting' -- it was GOD who fought; what the people did was to trust him and obey in faith... of which the singing was an expression.)

Answer 2

I am not sure. I have to research my bible for the answer.