User blog comment:B214/Chapter 383 Surfing Lucy thoughts and analysis/@comment-220.255.101.99-20140509140545/@comment-7418318-20140510171946

Gemini summoning another spirit is not the same as Angel summoning two spirits are the same time.

As for Layla, at the point she gave her keys away she had already weakened to the point that she could no longer be an active mage, so it's far more likely that she asked the three other people she gave the keys to for save-keeping (Zoldeo and the other two) to summon the Spirits so she could say her farewells.

And saying that summoning three isn't all that impressive compared to two is just nonsense. Just look at the difference between single and dual summoning. Sustaining two Spirits at the same time is far more exhausting than calling the two in a row, thus we can savely assume that the same would hold true for a third spirit.

There must be a good reason why everyone knowing about Celestial Magic is awed by "just" dual summoning and why the Spirits are worried that Lucy's body won't be able to take the strain of summoning three Zodiacs simultaneously.