A rigid block of mass $m$ on a horizontal surface is connected to three springs each having spring constant $k$ as shown in the figure.
Which one of the following is the CORRECT natural frequency of the system?

- $\sqrt{\frac{3 k}{4 m}}$
- $\sqrt{\frac{2 k}{3 m}}$
- $\sqrt{\frac{3 k}{2 m}}$
- $\sqrt{\frac{3 k}{m}}$