how to make a collection of multiple Custom control in c#

To use them in a loop, they have to be part of a collection - so either you need to insert them into a collection you create, or use the Controls collection of the form (or other control you added them to) to locate them.

With your own collection that is trivial, but teh Control collection needs more care:

foreach (Control c in Controls)
   {
   MyUserControl uc = c as MyUserControl;
   if (uc != null)
      {
      Console.WriteLine(um.MyProperty);
      }
   }

If the three controls are not all the same class, then you either need to check for all three, or for the base class for all three.

Note that if the user controls are inside a container such as a panel, you need to explicitly check that controls Controls collection.