User blog comment:MagnoCosmos/Celestial Spirits Ranking/@comment-1153699-20150907164700/@comment-1153699-20150907221449

She can summon 3 spirits. But how useful would it really be in battle though? Having 3 spirits out at once means she'll lose Magic rapidly. The more Magic one loses the weaker he/she gets (which means her spirits get weaker and weaker every second just by being around).

How long would she be able to have 3 spirits out casting spells and actively fighting against an enemy before she exhausts herself? 10 seconds? 20 seconds? 30 seconds? I doubt it would be a game changer. Sure, she'll momentarily have the upper hand against someone like non-PoF Juvia or Wendy, but as soon as Lucy runs out of Magic, it's over.

We also don't really know how much Magic 3 spirits really take. How much Magic would it take from Natsu or Gray? We don't know. For all we know, Natsu could have enough Magic to open 12 spirits at once effortlessly if he was in Lucy's shoes.