Choice A is the correct answer. The leading coefficient $a$ determines the vertical stretch or compression.

1. Here $|a| = 3$.
2. Since $|a| > 1$, the graph is vertically stretched away from the x-axis.
3. This makes the parabola appear narrower.

Choice B is incorrect because that happens when $0 < |a| < 1$. Choice C is incorrect because that would be $x^2 + 3$. Choice D is incorrect because that would be $(x - 3)^2$.