There's just one word in the English language with three ‘y's. That word is syzygy – pronounced siz-uh-jee. It's a word that astronomists would most likely be familiar with, and possibly even poets.
The Macquarie Dictionary lists one definition as “the conjunction or opposition of two heavenly bodies; a point in the orbit of a body, as the moon, at which it is in conjunction with or in opposition to the sun.” Other references suggest syzygy actually describes the alignment of three celestial bodies – something that happens when there's a full or new moon.
The unique thing about the definition of syzygy is that it describes both opposition and conjunction with the sun. The original meaning of the word only applied to conjunctions – when the moon is between the Earth and the sun. It came from the Latin suzugia and the Greek suzugos, which meant “yoked or paired”.
Somehow, it later came to mean opposition as well, or the state when the Earth is positioned directly between the moon and the sun, thus placing the moon and sun on opposite sides of the sky, giving us a full moon.
There is another, much rarer, definition of syzygy. It's also a “Classical Prosody” or “a group or combination of two feet (by some restricted to a combination of two feet of different kinds).” Huh?
I don't often have to check the definition of a definition of a word, but did have to with this one. Prosody is the study of poetic metres and versification and a syzygy in this sense is the combination of “two metrical feet into a single unit”. I'll let the poets figure out exactly what that means.
It's difficult to find good examples of syzygy in a sentence but I did find this lovely poem on O Dock:
“I think that I shall never see
A word as troubling as syzygy.
Though it may speak of celestial alignment
Speaking it’s a tough assignment.
The s and z, and then the g
Are in too close proximity
For tongues to tackle tactfully,
Too tight together to try, these three.
And having three (or just two) y’s
In such cramped space is none too wise.
Poems are made by fools like me
But only God can say syzygy.”