Program:

    #include<iostream.h>
    #include<conio.h>

    class one
    {
    private;
    int a,b,c;
    public:
    void enter(void)
    {
    cout<<"\n Enter the value of a,b and c:\n";
    cin>>a>>b>>c;
    }
    friend int largest(one big)
    };
    int largest(one big)
    {
    int max;
    max=big.a;
    if(big.b>max)
    max=big.b;
    if(big.c>max)
    max=big.c;
    return(max);
    }
    void main()
    {
    one obj;
    int result;
    clrscr();
    obj.enter();
    result=largest(obj);
    cout<<"\n Largest of three number is: "<<result;
    getch();
    }