Why did he split into three camps?
Malbim: It was so they could attack from three directions. 1
Gid'on said (verse 17) "see what I do, and do the same." Perhaps after they saw, the other two groups went to attack from other directions. (PF)
Why did he give to them Shofaros, and torches in empty flasks?
Rashi #1: The Shofaros and flames were to mention the merit of Matan Torah.
Rashi #2: It was night. They carried torches for light. They were in flasks so [the enemy] would not see the torches [until they broke the flasks - verse 19].
Malbim: The Teru'ah of war would make the enemy panic, and also the flames and smoke. The torches were in empty flasks so they will be seen suddenly, and they will hear the noise of the breaking flasks.
