While "yea" and "yeah" are also used in casual conversation, they are not the preferred spellings for an exclamation of joy.
* Yay is specifically used to express excitement, happiness, or success.
* Yea is often used as a short form of "yes," while
* Yeah is a more informal version of "yes."
So, if you're looking for the most standard and appropriate spelling for an exclamation of joy, yay is the best choice.