In the context of the swing set, the vertex of the parabola represents the maximum height that the swing seat reaches when it is at its highest point. Since the seat is 3 feet off the ground, this indicates the maximum height relative to the ground.
Therefore, the y-value of the vertex, in terms of this context, would be:
y = 3
This signifies that the highest point the seat can reach is 3 feet from the ground.