Answer
260 Hz.
Work Step by Step
For a guitar string, the frequency of the nth harmonic is $f_n=nf_1$. We see that the fundamental for this string is $f_1=f_3/3=(540\;Hz)/3=180\;Hz$.
When the string is fingered, it has a new length that is 0.70 multiplied by the original length.
The fundamental frequency of the vibrating string can also be calculated by using $f_1=\frac{v}{2\mathcal{l}}$. For this situation, v is constant (because the string tension has not changed).
$$f_{1,new}=\frac{v}{2\mathcal{l}_{new}}=\frac{v}{2(0.70)\mathcal{l} }=\frac{1}{0.70}f_1$$
$$ =\frac{1}{0.70}(180\;Hz)\approx260\;Hz$$
