Answer
The charges +2q and +q are enclosed by the Gaussian surface.
Work Step by Step
According to Gauss's law, the electric flux through any closed surface is directly proportional to the amount of charge enclosed by it. We are given that the electric flux is $\frac{+3q}{\epsilon_{\circ}}$, so the charges +2q and +q are enclosed by the Gaussian surface (and the negative charge is not enclosed).
