get array of last 3 dates with carbon

Try to parse() it first:
$date = Carbon::parse($dateTime)
            ->addSeconds($seconds)
            ->format('Y-m-d H:i:s');

$period = CarbonPeriod::create('2018-06-14', '2018-06-20');

// Iterate over the period
foreach ($period as $date) {
    echo $date->format('Y-m-d');
}

// Convert the period to an array of dates
$dates = $period->toArray();