Could anyone tell me how to find the charge when the element are in group 3-13?
I assume that your question is about the transition metals and the oxidation states(or charge on the ion) they exhibit in their compounds.
In that case, there is no specified rule. These elements can show multiple oxidation states. For instance, Mn can show +2,+3,+4,+6,+7 , Cr can show +2,+3,+6.
In some cases, it is reaction dependent.