You can add extras to an intent, for Activity1, use the following snippet:
Intent intent = new Intent(this, Activity2.class);
intent.putExtra("SuperSecretValue", /* put your string here */);
startActivity(intent);
In Activity2, use the following snippet:
String superSecretValue = getIntent().getStringExtra("SuperSecretValue");
Intent intent = new Intent(this, Activity3.class);
intent.putExtra("SuperSecretValue", superSecretValue);
intent.putExtra("AnotherSuperSecretValue", /* put your string here */);
startActivity(intent);
In Activity3, use the following snippet:
String superSecretValue = getIntent().getStringExtra("SuperSecretValue");
String anotherSuperSecretValue = getIntent().getStringExtra("AnotherSuperSecretValue");
Check the documentation for more information on what you can do with intents. For this specific case, you can add extra key, value pairs (called "extras") to the intent object that can be retrieved by name in the activity that the intent object is used to start.