To transform the function \( f(x) = |x^2| \) by vertically stretching it by a factor of 3, we multiply the entire function by 3.
This gives us:
\[ g(x) = 3|x^2| \]
Now, we can compare this with the provided options:
- \( f(x) = x^2 - 3 \)
- \( f(x) = x^2 + 3 \)
- \( f(x) = 3|x^2| \)
- \( f(x) = |(x + 3)^2| \)
The correct equation of the transformed function, which vertically stretches the original function by a factor of 3, is:
\[ \boxed{f(x) = 3|x^2|} \]