Answer
${\bf 1.05}\;\rm mm$
Work Step by Step
We know that the magnetic field created by a solenoid is given by
$$B=\dfrac{\mu_0NI}{l}$$
where $N$ is the number of turns which is given by the total length of the wire divided by the diameter of the wire.
$$B=\dfrac{\mu_0lI}{lD}$$
$$B=\dfrac{\mu_0I}{D}$$
Hence, the wire's diameter is given by
$$D=\dfrac{\mu_0I}{B}$$
Plug the known;
$$D=\dfrac{(4\pi \times10^{-7})(2.5)}{(3\times 10^{-3})}$$
$$D=\color{red}{\bf 1.05}\;\rm mm$$
