Also you skipped the pelican
The pelican just simply isnt in my top 3.
Ofcourse i want a pelican, but if its just 3 birds my top criteria are range and biome diversity.
The pelican isnt competing with the ducks here, the mallard is imo the nr 1 most important bird to be added, not because it itself is particulary exciting (i think most of us agree that the mandarine would be) but because it exist everywhere where a puddle can form. Its also not like they are ugly, both male and female are very visually appealing in their pattern and colors and if they wouldnt be literally everywhere the green and blue feather would be seen as colorfull and beautifull instead of mundane.
The White faced whistling Duck beautifully contrasts the mallard allmost perfectly.
They are beautiful and colorfull aswell, but very different both in pattern and colors, live in pretty much all of subsaharan africa, madagascar and warm south america and are a whistling duck, which is either a goose or its own thing, taxonomy isnt quite sure on that one, while also being very commonly held in zoos.
Together both species cover pretty much the entire world, giving a suitable waterfowl to literally everywhere, while also being very distinct from another so they do not devalue each other. Perfectly synergistic combo
Lastly the white stork would be the one id be most open to sub for a pelican, but i personally prefer the white stork. They cover everything the pelican does as an iconic bird from africa and india, while also covering europe, anatolia and central asia aswell. The most important factor would be biome and habitat design though.
Pelicans, as awesome as they are, are a distinctly aquatic animal and while i defenetly have seen creative pelican displays, for example in the zoom zoo where they keep pelicans in the water moat of the giraffe savannah habitat, the stork simply offers more options. They are just as fitting for habitats with water, be it marshes, riversides, floodplains whatever, but they are also grassland animals, be it the african savannah, european farmland or just a green meadow.
I get that pelicans are the goat of missing birds that isnt a duck, but it doesnt feel like a back bone species to me like these 3 do, but its defenetly part of the top 5 together with the victoria crowned pidgeon