Answer
36 Hz, 48 Hz, and 61 Hz.
Work Step by Step
The walls of the room are displacement nodes. The length, width and height each correspond to a half-wavelength of sound. The fundamental frequency for a standing wave in each case is $f=\frac{v}{2\mathcal{l}}$.
Length:
$$f=\frac{343m/s}{2(4.7m)}\approx 36\;Hz$$
Width:
$$f=\frac{343m/s}{2(3.6m)}\approx 48\;Hz$$
Height:
$$f=\frac{343m/s}{2(2.8m)}\approx 61\;Hz$$
