 Solution

Here three means the total spacetime dimension. A dimensionless action requires $[\mathcal{L}]=3$ in mass dimensions. The kinetic term gives $2+2[\varphi ]=3$, so $[\varphi ]=1/2$. Matching the mass dimensions of both field interaction terms givesThus $g_1$ has units of mass and $g_2$ is dimensionless. The quartic interaction is superrenormalizable by power counting, and $g_2$ is a marginal coupling by power counting.