public class A
{
	public Class1 a1 {get; set;}
	public Class2 b1 {get; set;}
	public Class3 с1 {get; set;}
}

public class B
{
	public A a {get; set;}
	public Class4 b2 {get; set;}
	public Class5 c2 {get; set;}
}

main()
{
	//Имя вложенного класса (a1 или b1 или с1)
	String subClassName = "a1";
	B b = new B();
	b.b2 = new Class4();
	b.c2 = new Class5();
	b.a = new A();
	//Здесь надо через рефлексию создать экземпляр субкласса по его имени subClassName 
	//для a1 - b.a.a1 = new Class1();
	//для b1 - b.a.b1 = new Class2();
	//для c1 - b.a.c1 = new Class3();
}

public object GetInstance(string strFullyQualifiedName)
{
      Type type = Type.GetType(strFullyQualifiedName);
      if (type != null)
           return Activator.CreateInstance(type);
       foreach (var asm in AppDomain.CurrentDomain.GetAssemblies())
       {
           type = asm.GetType(strFullyQualifiedName);
           if (type != null)
           return Activator.CreateInstance(type);
       }
       return null;
}

public object GetInstance(string strFullyQualifiedName)
{
      Type type = Type.GetType(strFullyQualifiedName);
      if (type != null)
           return Activator.CreateInstance(type);
       foreach (var asm in AppDomain.CurrentDomain.GetAssemblies())
       {
           type = asm.GetType(strFullyQualifiedName);
           if (type != null)
           return Activator.CreateInstance(type);
       }
       return null;
}

private B CreateIns(В b, String subClass)
{
...
}

B b = new B();
b.b2 = new Class4();
b.c2 = new Class5();
b = CreateIns(b, "a1");

B.a=new A();
B.a.a1= CreateInst("n1") as A;
B.a.a2= CreateInst("n2") as A;
B.a.a3= CreateInst("n3") as A;

b.GetType().GetProperty("a1").SetValue(b,CreateInst("a1"));