I would say quadruple because most of our DoTs include an initial attack. Not to mention that a lot of our spells are also aoes, so we'd need a mass quadruple prism, which would probably cost 5 or 6 pips.
This is why people have suggested Prism Blades in the past. A blade that you put on that converts the whole spell to the school opposite yours. For any spell that does more than one hit, this would be crazy helpful.
Triple prisms would be a good idea as traps are one of Fire's specialties...but there would be downsides to this spell relating to PvP. I know that if this were to become an actual Fire spell in the future, the PvP community would not take it so lightly. Shields would be rendered useless for the other schools if a Fire were to place triple prisms on their opponent and Fire would have an overwhelming advantage over the others. But if the spell were to be available for every school, I'd say they'd be more tolerable towards it.