Answer
$203\;\rm Hz$
Work Step by Step
We know that the beat frequency is given by
$$f_{\rm Beat}=f_1-f_2$$
Thus, the new frequency is
$$f_1=f_{\rm Beat}+f_2=3+200 $$
$$f_1=\color{red}{\bf 203}\;\rm Hz$$
It is a reasonable result since the increase in the tension increases the wave speed, and hence the frequency increases.
