steps for creating simple android application for addition of three number
Steps:
1. file->new->android application project
2. give Application name Addition
3.. Select minimum Required SDK and Target SDK and click on next select Blank activity and click on finish

4. In MainAcitvity write following code

import com.example.addition;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {

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

        EditText a=(EditText)findViewById(R.id.editText1);
        EditText b=(EditText)findViewById(R.id.editText11);
        EditText c=(EditText)findViewById(R.id.editText111);
   
             String s1=a.getText().toString();
        String s2=b.getText().toString();
        String s3=c.getText().toString();
        Integer i1=Integer.parseInt(s1);
        Integer i2=Integer.parseInt(s2);
        Integer i3=Integer.parseInt(s3);
        Integer add=i1+i2+i3;
        final String ans=add.toString();
        Button c=(Button)findViewById(R.id.button11);
        c.setOnClickListener(new View.OnClickListener() {
       
            @Override
            public void onClick(View v) {
           
                Toast.makeText(getApplicationContext(), ans, Toast.LENGTH_LONG).show();
            }
        });

   
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Step 5. write following code in activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.addition.MainActivity" >

    <EditText
        android:id="@+id/editText11"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView1"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="144dp"
        android:ems="10" >

        <requestFocus />
    </EditText>

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/editText11"
        android:layout_alignLeft="@+id/editText11"
        android:layout_marginBottom="64dp"
        android:ems="10" />

    <Button
        android:id="@+id/button11"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/editText11"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="76dp"
        android:text="Answer" />

    <EditText
        android:id="@+id/editText111"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:ems="10" />

</RelativeLayout>

Step 6. Save the project and click on run...