Hedwall, winless on the LPGA Tour, was a record 5-0 last week in Colorado in Europe's blowout victory over the United States. The Swede had a 10-under 200 total at Royal Mayfair.
Pettersen shot a 65, and the 16-year-old Ko had a 67. Last year in British Columbia, the New Zealand amateur became the youngest winner in LPGA Tour history at 15 years, 4 months.
Hedwall had seven birdies and a bogey in the third round after opening with consecutive 68s.
Brittany Lincicome and I.K. Kim were two strokes back at 8 under. Kim had a 65, and Lincicome, the 2011 winner at Hillsdale in Quebec, shot 66.
Top-ranked Inbee Park, tied for the second-round lead with Cristie Kerr, had a 74 to fall six strokes back at 4 under. Park swept the first three majors of the season and had six tour victories this year.
Kerr was another stroke back after a 75.