WHEN-VALIDATE-RECORD at Block level (Emp).
Declare
N Number;
Begin
Select count(*)   Into N from Emp_Table 
where Col1=:Emp.Col1
and   Col2=:Emp.Col2
and   Col3=:emp.Col3;

If Nvl(N,0)>0 then
message('Duplicate Value Exist !');
message('Duplicate Value Exist !');
Raise Form_Trigger_Failure;
End if;

End;