‘ And the children of Israel went up against the children of Benjamin on the third day, and set themselves in array against Gibeah, as at other times.'
This was the third day of battle not the third day in succession. (Alternatively it might be seen as the third day following the previous battle). There were three memorable days of battle. This was the third of them. The number three is the number of completeness and this indicated to them that God's perfect plan was coming to completion.
“And set themselves in array against Gibeah, as at other times.” They appeared to be following the same plan as previously. But this time with their full force (Judges 20:26). The children of Benjamin no doubt thought that their luck was in. These foolish children of Israel would never learn. While the numbers of their opponents had considerably increased (although they may not have been aware of this. Not all the units were openly deployed. See Judges 20:33), they could not all advance together, and they were probably confident that their slingers would again cause havoc.