Question

why O has valency 3 in fe3 O4​

Answer 1

oxide ions tends to have a 2-charge Fe3O4 is a neutral molecule.....

so that means that overall charge balance is

3 (Fe) + 4 (-2) = 0

which gives 8/3

but you can't have a fractional valency.( you can only lose a whole no. of electron, but not fraction of an electron ) so, this is only an average valency for the iron atoms in Fe3O4........☺☺☺hope it will help