Leading overnight by three shots after a second-round 64, Lewis had three straight bogeys in the front nine before the American fired a five-foot eagle on the 15th hole to regain the lead from South Korea's Yang.
Third-ranked Lewis had five bogeys - including one at the 18th - and two birdies at the Siam Country Club Pattaya Old Course for a three-day total of 13-under 203.
Sandra Gal of Germany shot 68 for third place, one stroke further behind Yang.
''It was a bit all over the place today,'' Lewis said. ''Hit some good shots, hit some shots too far.''
Yang bogeyed Nos. 12 and 14 before back-to-back birdies on the 17th and 18th holes saw her finish with a 71.