Первое действие при нажатии кнопки
intent = new Intent(FirstActivity.this, SecondActivity.class);
intent.putExtra("firstno", String.valueOf(firstnumber));
intent.putExtra("secondno", String.valueOf(secondnumber));
intent.putExtra("thirdno", String.valueOf(thirdnumber));
startActivity(intent);
Второе действие при загрузке
String firstNo, SecondNo, ThirdNo;
firstNo = getIntent().getExtras().getString("firstno");
SecondNo = getIntent().getExtras().getString("secondno");
ThirdNo = getIntent().getExtras().getString("thirdno");
Отредактированный контент
Double firstNo, SecondNo, ThirdNo;
firstNo = Double.parseDouble(getIntent().getExtras().getString("firstno"));
SecondNo = Double.parseDouble(getIntent().getExtras().getString("secondno"));
ThirdNo = Double.parseDouble(getIntent().getExtras().getString("thirdno"));
textView.setText(String.valueOf(firstNo + SecondNo + ThirdNo));