How to check if multiple checkboxes are checked

So in my program, i have three checboxes (A, B and C). and I want to save the content of the checkbox the is checked to a text file. I am doing this using if statements as shown below:

        if (a.IsChecked == true)
        {
            res = a.Content.ToString() + " is checked"; 
        }
        else if (b.IsChecked == true)
        {
             res = b.Content.ToString() + " is checked";
        }
        else if (c.IsChecked == true)
        {
             res = c.Content.ToString() + " is checked";
        }

And here is where i am saving the above values to a string and then later in my code to a text file

       string test= res;

Now this is working for me. So i decided to try to check if multiple checkboxes are being checked. So added the below if statements:

     else if ((a.IsChecked == true) && (b.IsChecked == true) && (c.IsChecked == true))
     {
      res= a.Content.ToString() + " " + b.Content.ToString() + " " + c.Content.ToString()
     }

but this isn't working with me because in the end res is printed in the text file as a rather than a b c. Any idea what i am doing wrong?

Also please note that i already initialized res at the top of my code as string:

    string res;

I am not getting any error when i run my code so i am not sure where my mistake is. any help with this is much much appreciated.

thanks a lot :)