Which function has a y-intercept of 3?

If you have choices, you should list them. If you are just asking about any example, then here are some.

y = a*x + 3 ; the value of a can be any real number.

- 3x + y = 3
 2y + 4x = 6

ny = ax + 3*n When you divide through by n, you get
y = a/n * x + 3n/n
y = ax/n + 3

Well if you have 20 bats you probably want 20 balls? you don’t have enough context to answer the question.