Schauffele and Morikawa, who will go out in the final pairing at 2:35 p.m. ET, are both 15 under on the week and one shot clear of Sahith Theegala, who is seeking his second PGA Tour win in his 90th start.
Former British Open champion Shane Lowry, whose electric 62 on Saturday matched the lowest round in major history, is a further shot back in a share of fourth place with LIV Golf's Bryson DeChambeau and Norway's Viktor Hovland.
According to Elias Sports Bureau, 12 of the last 13 PGA Championships were won by a player who was leading or within two shots of the 54-hole lead, the lone exception coming in 2022 when Justin Thomas came back from seven shots.
Englishman Justin Rose, who has recorded five consecutive top-15 finishes at the PGA Championship, is three shots back of the leaders and sharing seventh place with Robert MacIntyre, with the duo going out together at 2:05 p.m. ET.
South Africa's Dean Burmester was alone in ninth place and four back of the co-leaders.
Olympic champion Schauffele, who held the solo lead after the first two rounds, is seeking his first major triumph while fellow American Morikawa is seeking a third after winning the 2020 PGA Championship and 2021 British Open.
Northern Irishman Rory McIlroy, who has been stuck at four majors since his 2014 PGA Championship triumph at Valhalla, will do all he can to snap his drought when he sets out at 12:45 p.m. ET seven shots back of the co-leaders.
Jordan Spieth, who breathed life into his bid to complete the career Grand Slam of golf's four majors this week with a 67 on Saturday to reach eight under on the week, will be a further group behind.
World number one Scottie Scheffler will head out at 12:15 p.m. ET eight shots back of the co-leaders after losing ground during a third round played a day after being arrested on four counts, including second-degree assault of a police officer.
Defending champion Brooks Koepka, who arrived at Valhalla looking to become the PGA Championship's first repeat winner since he successfully defended the title in 2019, went out with the early starters a distant 11 shots back of the lead.
