Matt Weibring (71), Chad Ginn (72) and second-round leader Jeff Klauk (73) joined Mathis at 9-under-par 207 for a one-shot lead over four other players.
Klauk lost sole possession of the lead twice down the stretch, once when a bogey at the 16th dropped him into a tie with Weibring, then when a double- bogey at the 17th put him one shot behind. He closed with a birdie at the 18th hole to join the other co-leaders.
Greg Chalmers fired a seven-under 65 to move one shot off their lead at 8-under 208 with Gary Christian (68), Ricky Barnes (70) and David Hearn (71).
Among the leaders, only Mathis and Klauk have won on the Nationwide Tour before, with both players holding one career victory (Mathis won last week's BMW Charity Pro-Am).
There were 19 players within five shots of their lead.
'It'll be a shootout,' said Weibring.
Weibring shared the lead with Klauk late in the round before he three-putted for a bogey at the 17th to fall one back. It was his second consecutive bogey and his third of the round. He also had two birdies and an eagle at the par- five sixth.
Klauk had collected three birdies and two bogeys before his double-bogey at the 17th and his closing birdie at the 18th.
'It should be pretty exciting tomorrow,' he said.
Mathis' round was less spotty. He had seven birdies with only one bogey on Saturday, the most significant part of his round being an eight-hole stretch between the fifth and 12th holes where he made six birdies. He closed with a birdie at the 18th to reach nine-under.
Ginn posted his modest 72 after collecting two birdies and two bogeys, including a bogey at the 16th that hurt his chances to hold the lead alone.
It was a tough day for most of the players, though not all of them expected it would remain that way.
'Whoever wins is going to have to shoot four- or five-under at least,' said Weibring. 'It won't happen two days in a row like this. Six or seven-under is out there.'