When resisting drain from a manabolt, the drain is always (Force/2), unless you're using that ridiculously craptacular optional rule that others will likely mention. If you look in the book, all the spells except Heal have a drain value of (Force/2)+X. Spellcasting hits have zilch to do with resisting drain. You could roll all sixes on the spellcasting test, and it won't do a thing to resist drain. And yes, 3 Drain doesn't sound like much, but when you're only rolling 8-12 dice to resist it, then you're at the edge of probabilities there. And with higher drain spells, casting them at high Force becomes a lot like shooting yourself in the foot.