Matthew recorded five birdies, including three in a row around the turn, before her first bogey of the tournament came at the 17th and left her on 11 under at the end of the day.
That was still enough for a comfortable lead over her nearest challenger Suzann Pettersen, the Norwegian moving to eight under after a 71 which saw her birdie the par-five third and 18th but bogey the 14th.
Last year's champion Kim In-kyung of South Korea shares third after a 70 left her one shot further back alongside Swede Anna Nordqvist, who could only manage a disappointing 73.
Overnight leader Juli Inkster's bid to become the LPGA Tour's oldest winner at 51 stalled as four bogeys and only a solitary birdie led to a three-over 75 and a three-round total of five under, six shots behind Matthew.
World number one Yani Tseng had a second disappointing round out of three, a 74 leaving her three over for the week.