What’s the name of the little widget with three dots inside a cardview in android?

This is not a widget at all. It is an ImageButton (borderless in style) using the overflow Icon that includes a PopupMenu

For documentation tutorial visits http://developer.android.com/guide/topics/ui/menus.html#PopupMenu

This refers to a nice code snippet from the link above:

  <ImageButton
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content" 
       android:src="https://stackoverflow.com/questions/31765395/@drawable/ic_overflow_holo_dark"
       android:contentDescription="@string/descr_overflow_button"
       android:onClick="showPopup" />

Then use to show popup:

 public void showPopup(View v) {
     PopupMenu popup = new PopupMenu(this, v);
     MenuInflater inflater = popup.getMenuInflater();
     inflater.inflate(R.menu.actions, popup.getMenu());
     popup.show();
 }

3-Dots button is available among Assets in Android Studio:

Right click on res -> New -> Vector Assets -> Asset Type = Clip Art

-> Click on the button next to Clip Art: label -> Search for more vert