Peterson stands at 7-under-par for the tournament. Jason Buha is in second place at 6-under following a Saturday 68.
Second-round leader John Elliott is now in third place after a 1-over-par 71 at the Glenmaura National Golf Club. Elliott led by as many as three shots in the third round, but posted two bogeys and no birdies over his final 14 holes.
Peterson, in his ninth season on tour, is vying for his first victory since the 1995 Central Georgia Open. He led by one shot through three rounds of this years Charity Pro-Am, but could only manage a Sunday 71 and tied for third.
Buha is a rookie on the Buy.Com Tour. The former Duke Blue Devil is coming off his best showing of the season, a tie for ninth at last weeks Richmond Open.
Full-field scores from the Buy.Com Steamtown Classic