$a)\frac{\left|x-a\right|}{\left|x\right|}=\frac{\left|3,1-3\right|}{\left|3,1\right|}=\frac{\left|0,1\right|}{3,1}=\frac{0,1}{3,1}=\frac{1}{31}=0,0322\dots \approx 0,032=3,2\%$

$b)\frac{\left|x-a\right|}{\left|x\right|}=\frac{\left|45,9-46\right|}{\left|45,9\right|}=\frac{\left|-0,1\right|}{45,9}=\frac{0,1}{45,9}=\frac{1}{459}=0,0021\dots \approx 0,002=0,2\%$

$c)\frac{\left|x-a\right|}{\left|x\right|}=\frac{\left|45,9-45\right|}{\left|45,9\right|}=\frac{\left|0,9\right|}{45,9}=\frac{0,9}{45,9}=\frac{9}{459}=0,0196\dots \approx 0,020=0,02=2\%$