List<Class1> list1 = new List<Class1>();
List<Class2> list2 = new List<Class2>();
List<Class3> list3 = new List<Class3>();

form.SetData(list2);

List<T> data= ... ???

public void SetData<T>(List<T> data)
{
    this.data = data;
}

namespace TestListT
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Class1> list1 = new List<Class1>();
            list1.Add(new Class1(1, "Apple"));
            list1.Add(new Class1(2, "Samsung"));
            list1.Add(new Class1(3, "LG"));

            List<Class2> list2 = new List<Class2>();
            list2.Add(new Class2(10, "Microsoft", "USA"));
            list2.Add(new Class2(15, "Yandex", "Russia"));
            list2.Add(new Class2(20, "Sony", "Japan"));

            MyForm form = new MyForm();
            form.SetData(list1);
        }
    }

    class MyForm
    {
        List<object> data;

        public void SetData(List<object> data)
        {
            this.data = data;
        }        
    }

    class Class1
    {
        public int ID { get; set; }
        public string Name { get; set; }

        public Class1(int id, string name)
        {
            ID = id;
            Name = name;
        }
    }

    class Class2
    {
        public int Num { get; set; }
        public string Name { get; set; }
        public string Address { get; set; }

        public Class2(int num, string name, string address)
        {
            Num = num;
            Name = name;
            Address = address;
        }
    }
}