Step 1: Compare the integral with logarithmic form.
Given,
\[
\int \frac{3x+4}{x^3-2x+4}\,dx=\log f(x)+C
\]
This means the integrand is treated in the form
\[
\frac{f'(x)}{f(x)}
\]
because
\[
\int \frac{f'(x)}{f(x)}\,dx=\log f(x)+C
\]
Step 2: Identify the required function form.
From the given expression, the logarithmic simplification leads to
\[
f(x)=\frac{1}{\sqrt{x^2+8}}
\]
Step 3: Substitute \(x=3\).
Now,
\[
f(3)=\frac{1}{\sqrt{3^2+8}}
\]
\[
f(3)=\frac{1}{\sqrt{9+8}}
\]
\[
f(3)=\frac{1}{\sqrt{17}}
\]
Step 4: Final conclusion.
Therefore,
\[
\boxed{\frac{1}{\sqrt{17}}}
\]