Ternary compounds are compounds made up of 3 or more different elements. It may be composed of a positive metal and a negative polyatomic ion (radical), or a positive polyatomic ion and a negative element or ion. Listings of polyatomic ions are usually contained in the Periodic Table. 

Writing formulas of ternary compounds follows the same fundamental steps as writing formulas of binary compounds. Special attention however should be considered in the polyatomic ions or radicals. A radical is a group of atoms which acts as a single atom. They do not separate with one another in simple or ordinary chemical reaction. That's why regardless of number of elements in a radical, they only have one oxidation state or valence.

Let us consider compounds formed by elements and radicals having oxidation states that are numerically equal and those that are not.

1. Sodium Acetate Sodium being the first element is positive and Acetate being the second as polyatomic ion or radical must be negative. Sodium is positive +1 and acetate is -1. The subscripts 2,3 and 2 in C, H and O are inherent of the elements in the radical, therefore they must be retained in the final formula of the compound. 

Sodium Acetate ------------- Na+1 C2H3O2-1

2. Calcium Phosphate Calcium is positive and Phosphate is negative. From the Periodic table, Ca has an oxidation state of +2 and PO4 -3 . The oxidation states are not numerically equal. 

Calcium Phosphate -------------- Ca3(PO4)2

In naming ternary compounds, the same rule applied only that the second element is a polyatomic ion and need to mention the name.