U.S. energy firms this week added oil and natural gas rigs for a third week in a row for the first time since February, energy services firm Baker Hughes said in its closely followed report on Friday.The oil and gas rig count, an early indicator of future output, rose by three to 542 in the week to September 19, its highest since July. Despite this week's rig increase, Baker Hughes said the total count was still down 46 rigs, or 8% below this time last year. Baker Hughes said oil rigs rose by two to 418 this week…