Triple Citrus?

Regine is right. I got the cookbook from which this recipe is excerpted, and it calls for "½ teaspoon freshly grated zest" of orange, lemon, and lime (so 1½ teaspoons between the three citruses).

Other similar recipes use an equal mixture of juices and zests from orange, lemon & lime. You could do the same, using this recipe's quantities and dividing according to taste, or what you have on hand.

These recipes really need to be edited, this kind of thing is getting far too common. Since it says "zests", I'd guess that they intended to zest the orange. The triple could refer to the zest, juice, and orange segments, I suppose, but the recipe is terminally ambiguous.

Must be lemon zest. I would add 1/2 tsp each of lemon zest and orange zest to the 1/2 tsp lime zest that is mentioned in instructions.