The 1d10W only applies to one of the targets.
Compare to the spell Lightning Bolt:
This spell is more powerful version of the Fire Ball spell, allowing the caster to throw bolts of lightning rather than fire. Only one lightning bolt per level may be thrown during the combat round. Each bolt must be thrown at the same individual or group. Target restrictions apply in the same way as for normal missile fire.
Lightning bolts have a range of 48 yards and automatically hit their target. If fired into a group, they will hit 1D3 creatures or characters per level of the Caster. Each hit causes 1D10 W at S 4 (irrespective of armour) and 1D8 additional wounds on flammable targets.
Notably, this spell does not have a statement restricting the 1d10W to a single target. So when fire ball says,
Each hit has a Strength of 3 and causes 1D10 Wounds on one target,
only one of the targets in the group takes the 1d10W, unlike lightning bolt, which deals 1d10W to each member of the group.