BETHESDA, Md. — In Gee Chun of South Korea rallied after losing the rest of her once-sizeable lead, overcoming a bogey-filled front nine to win the Women’s P.G.A. Championship on Sunday when Lexi Thompson faltered with her putter.Chun shot a three-over-par 75 for the second consecutive day at Congressional Country Club outside Washington, D.C., but that was enough for her to win her third career major title by a stroke over Thompson, an American, and Minjee Lee of Australia. Chun, after leading by six strokes at the tournament’s midway point, lost a three-shot advantage in the first three holes of the final round. Thompson led her by two strokes after the front nine, but Thompson’s putting problems were just beginning.Thompson, 27, botched a par putt from a couple of feet on No. 14, but a birdie on the 15th hole restored her lead to two strokes. Then she bogeyed the par-5 16th hole while Chun birdied, leaving the players tied with two holes remaining.Thompson three-putted for bogey on No. 17, and after an impressive approach from the rough on the 18th hole, her birdie putt wasn’t hit firmly enough.Chun’s approach on the par-4 18th bounced past the hole and just off the back of the green, but she putted to within about 5 feet and sank her par attempt for the win.Chun, 27, led by seven strokes after finishing her round with an eight-under-par 64 in wet conditions Thursday. Her lead was down to five at the end of that day — still equaling the largest 18-hole advantage in the history of women’s major tournaments.She was six strokes ahead at the halfway point and had a three-shot advantage coming into Sunday. She finished the tournament with a 283, five under par.Chun won her first major at the U.S. Women’s Open in 2015 and added the Evian Championship in France the following year.Thompson hasn’t won an L.P.G.A. Tour event since 2019, and her lone major victory came as a teenager at Mission Hills in the California desert in 2014. She has certainly had chances: She lost a five-stroke lead during the final round of last year’s U.S. Women’s Open at The Olympic Club in San Francisco.This year, she was 10 strokes back after the first round before steadily chasing down Chun. Thompson made birdies on Nos. 1 and 3 on Sunday. Chun bogeyed Nos. 2 and 4 to fall out of the lead.Thompson missed short birdie putts on the eighth and ninth holes — foreshadowing her problems later in the round — but Chun’s 40 on the front nine left her two back at the turn. Sei Young Kim, who had made it to six under at one point, bogeyed holes eight, 10, 11 and 12 and fell out of contention. She finished the tournament in a five-way tie for fifth.When Chun made her first birdie of the day at the par-5 11th, Thompson answered with a birdie of her own to remain two shots ahead at seven under. When Thompson bogeyed 12, so did Chun.The 16th hole, where Chun had to take an unplayable lie and made double bogey Saturday, was the turning point in her favor in the final round. Thompson was just short and right of the green in two shots but took four from there to make bogey, while Chun rolled in her birdie putt after a long wait.