$a)$  

$\frac{3}{10}=0,3$  

$\frac{7}{10}=0,7$  

$1\frac{9}{10}=1,9$  

$3\frac{1}{10}=3,1$  

---

$b)$  

$\frac{4}{100}=0,04$  

$\frac{21}{100}=0,21$  

$3\frac{17}{100}=3,17$  

$5\frac{9}{100}=5,09$  

---

$c)$  

$\frac{7}{1000}=0,007$  

$\frac{341}{1000}=0,341$  

$\frac{39}{1000}=0,039$  

$4\frac{17}{1000}=4,017$