[+1]: Draw a card. [–2]: Ral, Caller of Storms deals 3 damage divided as you choose among one, two, or three targets. [–7]: Draw seven cards. Ral, Caller of Storms deals 7 damage to each creature your opponents control.
If some of the targets become illegal for Ral's second ability, the original division of damage still applies, and the damage that would have been dealt to illegal targets isn't dealt at all.