"Then Joshua made a treaty of peace with them to let them live, and the leaders of the assembly ratified it by oath. Three days after they made the treaty with the Gibeonites, the Israelites heard that they were neighbors, living near them." NIV translation
Joshua did not pray before making a decision and the other leaders agreed with him without seeking God's will in the matter. Three days after making the oath, they heard that they had been deceived. Throughout the Bible, the number three is associated with the earthly display of God's will for men.
At the end of these days, they found that they were out of God's will in the matter but they had made a promise that they could not break.
As Christians, this period of three days would have been a good time for prayer and fasting to seek the will of God. This is how we are to face the schemes of the devil without falling for them.
"So the Israelites set out and on the third day came to their cities: Gibeon, Kephirah, Beeroth and Kiriath Jearim. But the Israelites did not attack them, because the leaders of the assembly had sworn an oath to them by the Lord, the God of Israel. The whole assembly grumbled against the leaders, but all the leaders answered, 'We have given them our oath by the Lord, the God of Israel, and we cannot touch them now.'" NIV translation
The Israelites went to confront the Gibeonites but did not attack them because of the treaty. The rest of the people were upset with the leadership but they could not break their oath. This is a reminder of what happens in any group that is not completely led by God.
This would have been a total of six days after they had made the decision. We are reminded that the number six is associated with the works of men and, now, they had come to the end of what they could do. We are reminded that they could have avoided all of this by praying on that first day. It is also better for us if we do not swear any oaths because the devil can use them against us.