How to access to common properties of a series of classes without reflection

I am working on a legacy C# application that includes many business entity classes with a 5 common properties same name same type (string and integer).

I need to implement some business process logic on the entity classes based on the 5 common properties.

class A
{
    public string CommonProperty1 {get;set;}
    public int CommonProperty2 {get;set;}
    public string CommonProperty3 {get;set;}
    public string CommonProperty4 {get;set;}
    public string CommonProperty5 {get;set;}

}
class B
{
    public string CommonProperty1 {get;set;}
    public int CommonProperty2 {get;set;}
    public string CommonProperty3 {get;set;}
    public string CommonProperty4 {get;set;}
    public string CommonProperty5 {get;set;}

}
class C
{
    public string CommonProperty1 {get;set;}
    public int CommonProperty2 {get;set;}
    public string CommonProperty3 {get;set;}
    public string CommonProperty4 {get;set;}
    public string CommonProperty5 {get;set;}

}
// there more of such business classes

public static BusinessHelpr
{
   public static DoSomethingOnClassAorBorC(object theRefrence)
   {
       theRefrence.CommonProperty4 = "result of some complex calculation";
       theRefrence.CommonProperty2 = 56; // result of some complex calculation;
       theRefrence.CommonProperty5 = "result of some complex calculation";
   }

}

If this was a greenfield application, I would inherit from a base class, that includes the 5 properties, and nicely implement the required logic

However, there is a decision that we do not do any refactoring or changing the business entities. They cannot be touched.

As such, In my helper class I need to find a way to get a reference to an object type, and access to its properties by the name of them in a string. An obvious option here is reflection. That means I get a type, use reflection to access its properties by name in a string. However, I learned that the use of reflection in this scenario imposes performance penalty and it is not a good practice.

Please note that I simplified the actual scenario to focus on the main point. so creating a method below won't work:

static DoSomethingOnClassAorBorC(string CommonProperty1, int CommonProperty2, string CommonProperty3,string CommonProperty4, string CommonProperty5)

What are my other options, other than reflection?