Pettersson's 9-foot birdie effort on No. 18 put him at 12-under 201, a single shot better than second-round co-leader Knost (2-under 69).
Alone in third place was Zach Johnson, who carded a 5-under 66 to end 54 holes at 8-under 205.
Grabbing sole possession of fourth at minus-7 was two-time Heritage winner Boo Weekley.
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