A practical setup is usually two strobes with large modifiers, not bare flashes. Use big umbrellas or softboxes to soften the light and make it more flattering.

Place one light in front of each end of the row, then aim each light so its coverage reaches from that end toward the middle. Raise the lights as high as practical—often around 10–12 feet for standing adults, or at least well above eye level—to reduce harsh shadows, red-eye, and glasses reflections.

Also move the lights back far enough that the distance difference from the lights to the people at the ends versus the middle is smaller. That helps keep exposure more even across the group. Make sure the modifiers and beam spread are wide enough to cover from each end to the center.

Adding a third light aimed at the center is usually unnecessary unless you still have coverage problems. If your two main lights are large enough, high enough, and far enough back, they should give more even and flattering illumination without over-lighting the middle.