Question

Name the compound mg3cl2 is it magnesium chloride?

Answer 1

No 
Magnesium Chloride is MgCl2 

The method I use to name ionic compounds is 'swap and drop' 

Mg oxidation number is +2 and Cl oxidation number is -1
Mg^2+         Cl^-1  
'swap'
Mg^1           Cl^2 
'drop' 
MgCl2