Can we use any specific design pattern here?

I currently have some functionality like following in my method.

if(obj instanceof A) {
            someString = ((ChildClass)((A)obj)).get(0).getSomeString();

        }
if(obj instanceof B) {
            someString = ((B)obj)).getSomeString();
        }
if(obj instanceof C) {
            someString = ((c)obj)).getSomeString();
        }

        someString = performSomeOperation(someString);

if(obj instanceof A) {
            ((ChildClass)((A)obj)).get(0).setSomeString(someString);
        }
if(obj instanceof B) {
            someString = ((B)obj)).setSomeString(someString);
        }
if(obj instanceof C) {
            someString = ((c)obj)).setSomeString(someString);
        }

I am advised to use builder pattern, but I am not sure if we can use builder pattern here, is it possible? if not, is there any other approach?