Set the default checked option for a select list field.

ninja_forms_render_options.php

<?php

add_filter( 'ninja_forms_render_options', 'my_ninja_forms_render_options', 10, 2 );
function my_ninja_forms_render_options( $options, $settings ){
  foreach( $options as &$option ){
    if( 'three' == $option[ 'value' ] ){
      $option[ 'selected' ] = 1;
    }
  }
  return $options;
}