civicrm_api3('Contact', 'get', array(
  'sequential' => 1,
  'return' => "group",
  'id' => 3,
));

[
  "is_error" => 0,
  "version" => 3,
  "count" => 1,
  "id" => 3,
  "values" => [
    {
      "contact_id" => "3",
      "groups" => "Example group,Example smart group",
      "id" => "3",
    }
  ]
]

[
  "is_error" => 0,
  "version" => 3,
  "count" => 1,
  "id" => 3,
  "values" => [
    {
      "contact_id" => "3",
      "groups" => "4,7",
      "id" => "3",
    }
  ]
]