Verplank, who bogeyed the 18th hole for the second consecutive day, shot a 3-under 69 to match Marino at 19-under 197. The pair is two shots ahead of Davis Love III (64) and Scott Sterling (66).
The Magnolia Course hosts the final two rounds after players competed with amateurs the first two days. Action was split between the Palm Course and the Magnolia Course.
This is the final PGA Tour event of the season, and players are fighting to get into the top 125 on the money list to keep their PGA Tour cards for next season.
Three of the top-4 players on the leaderboard are safe for next year. Sterling is 153rd on the money list and needs a huge Sunday if he is to remain on Tour in 2009.
The other big story this week is that of Erik Compton, who is playing on a sponsor's exemption. Compton is five weeks removed from his second heart transplant and is availing himself nicely.
Compton managed only an even-par 72 on Saturday but is tied for 49th place at 6-under 210.
'Anybody in my situation takes it second by second,' said Compton. 'I'm just enjoying today, and I'll work a bit on the driving range and come back tomorrow and try to make a lot of birdies.'
Marino has made a lot of birdies this week ' 19 in fact ' and has yet to make a bogey through three rounds.
'Hopefully I can keep that up,' said Marino, who is winless on the PGA Tour. 'I don't know if I can go 72 holes without making a bogey. That's a tall order.'
Marino tried to make up ground quickly on Saturday. He trailed Verplank by three to start the third round but rattled off back-to-back birdies at one and two.
Verplank birdied the third, and both golfers birdied the par-5 fifth, although Marino had less than 20 feet for eagle. Verplank rolled in a 14-footer for birdie at the ninth, and Marino birdied 10 to get within two of Verplank.
At the par-4 11th, Verplank drove into the right rough but hit a spectacular second to 3 feet. He kicked in the birdie putt and moved ahead by three.
Marino drained a 7-foot birdie putt at the 13th, but Verplank had slightly closer to maintain his three-shot cushion. Verplank's putt lipped out on the left side, leading to some more problems at the next hole.
Verplank hit his second at the par-5 14th far and into the water on the left. He made a 5-footer to save par, but Marino tapped in a 3-footer for birdie to draw within one.
Both players saved nice pars from bunkers at the 17th, and neither was in great shape at the closing hole. Marino was on the back of the putting surface and lagged to three feet. Verplank found more sand and did all he could to blast out to 12 feet.
Verplank missed the par save, and Marino sank his to knot the two atop the leaderboard.
For Verplank, that was the second bogey in as many days at the 18th.
'I'm fine,' said Verplank, who has battled a cold all week. 'I played good today. It wasn't quite as good today, but I'm doing a lot of good things. I'm putting good and driving it very straight.'
Verplank shared the 54-hole lead last year and tied for third. In 2003, he was tied for the third-round lead and shared runner-up honors, but he has an experience advantage over Marino.
Verplank has five PGA Tour victories, including last year's Byron Nelson, while Marino has none. Verplank has held a piece of the 54-hole lead several times on tour and this is the first for Marino.
Tommy Gainey (66), Robert Garrigus (68) and Troy Matteson (69) share fifth place at minus-16. Matteson is the only player of the group safely inside the top 125.