The Englishman, who is yet to win on the European Tour, took the outright lead with birdies at No. 9 and No. 11 before play at the Green Eagle Golf Course was suspended for almost three hours due to lightning storms.
McEvoy parred his remaining holes while DeChambeau birdied the last to retake a share of the lead at 12-under heading into the final day.
"It was good," McEvoy told the European Tour's official website. "I was a little disappointed with the back nine, missed a few chances coming in on the last four holes or so but the change in conditions made it quite tough."
"I've been playing very solidly, I'm enjoying being at the top of the leaderboard which is great and I'm not putting too much pressure on myself."
For DeChambeau, however, the change in conditions signified an upturn in fortunes.
"On the front nine I had nothing," he said. "I was so happy the storm came through because that gave me a little time to get the game back in order and I was able to do that and play well on the back nine."
Paul Casey moved up four places to fifth after also shooting 3-under and is now 9-under overall.