It would be -60+loopindex*30.
Each fireball's angle gets set in the same loop as it was created.
"loopindex" for the first loopindex is 1, for the second it's 2 and for the third it's 3.
This means that for the first fireball created, the calculation would be -60+1*30 which is -30, exactly what we want. The second one would be -60+2*30 which would be 0. And so on.
Now to make this simplier, you could exchange the "repeat" condition by a "for" condition, in this case "For 0 to 2".
You would then change the "Set angle" action to exactly what you proposed, -30+loopindex*30, since the first loopindex would now be 0 and the last one 2.
Choose whatever you prefer. :)
(Remember: "Repeat" always starts at loopindex 1 while "For" makes you able to set where to start and where to end.)