Q:

Button does nothing

I am trying to make a button that goes from activity 2 to activity 3.
I already made a button that goes from activity 1 to 2 and that worked perfectly. When I press the button in the app it does nothing. This is my code so far. 
This is Main2Activity.java
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Main2Activity extends Activity {
Button button;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);

    button = (Button) findViewById(R.id.btnRemote);
    button.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent myIntent = new Intent(Main2Activity.this, Main3Activity.class);
            startActivity(myIntent);
        }
    });
  }
}

And this is my activity_main2.xml
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="uk.co.ndevr.evewebapp20.Main2Activity">

<Button
    android:id="@+id/btnRemote"
    style="@style/Widget.AppCompat.Button.Colored"
    android:layout_width="164dp"
    android:layout_height="45dp"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:backgroundTint="@color/colorPrimary"
    android:text="TV Remote"
    android:textStyle="bold"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.082"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.013" />

</android.support.constraint.ConstraintLayout>

AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="uk.co.ndevr.evewebapp20">

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".Main2Activity"
        android:label="@string/title_activity_main2"
        android:theme="@style/AppTheme" />
    <activity
        android:name=".Main3Activity"
        android:label="@string/title_activity_main3"
        android:theme="@style/AppTheme"></activity>
</application>

</manifest>

Main3Activity.java
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Main3Activity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main3);
  }
}

A:

You are never actually switching to MainActivity2 in the first Activity.
Calling setContentView() only changes the layout file inflated for the current Activity. This means, you have never called Activity2 so the onCreate() of your 2nd Activity is never called and the OnClickListener for that button is never registered.
In your onClick() of your first Activity, change from calling setContentView() (you almost never want to do this more than once in an Activity) to using an Intent as you are doing in the second Activity.

