The length of a rectangular field is three times its breadth.

Let the length and breadth of the rectangle be $3 x \mathrm{~m}$ and $x \mathrm{~m}$, respectively.

According to the question:

$3 x \times x=147$

$\Rightarrow 3 x^{2}=147$

$\Rightarrow x^{2}=49$

$\Rightarrow x=7$ or $x=-7$

$\Rightarrow x=7 \quad(\because x$ cannot be negative $)$

$\therefore$ Length $=3 \times 7=21 \mathrm{~m}$

Breadth $=7 \mathrm{~m}$