What is y when x is three?

This question cannot be answered without the whole equation:

If the equation is y = 2x, then y = 6 for x = 3.

If the equation is y = x + 2, then y = 5 for x = 3.

If the equation is y = 99/x, then y = 33 for x = 3.

If you can provide the complete equation, your question could be answered more precisely.