“First three songs and no flash” is the catchall restriction applied to professional photographers given passes to take photos from the pit (the space in front of the stage between the stage and the crowd safety barrier, if there is one). This basically means that after three songs, the photographers must exit the pit and take no more photos. They also cannot use a flash while taking photos in the pit.
The reasons for this restriction are twofold. Firstly, you will become hot under the stage lighting and may have become sweaty and dishevelled. That is not a good look for pictures that are going to be featured in official blogs, national newspapers or magazines.
Secondly, most musicians onstage are playing some kind of instrument that usually requires them to look down at their instrument (guitar, bass, keyboards, strings, and woodwind, for example). Whereas the stage lighting rig will be focused to shine on the musicians from above and behind, a photographer’s flash gun will be aiming straight at you, from below. You may therefore become slightly distracted when studying your fret board and some photographer blasts you from the pit with her mega-camera flash.
Beside all that, photos taken with flash look lousy.