Lyle fired a four-under 67 at Tres Marias Golf Club to join Chalmers at 9-under 204.
Tom Johnson posted a one-under 70 on Saturday and is tied for third place with Matthew Every, who carded an even-par 71. The pair finished 54 holes at minus- 8.
Chalmers began the third round with a one-shot lead, but a bogey at the third kept him atop the leaderboard. Lyle flew up the board with four birdies through seven holes to take first, but Chalmers matched him with his birdie at No. 7.
Both players bogeyed eight, but Chalmers birdied the ninth to move back into sole possession of the lead, but now it was by one over Every. When Every bogeyed 11 and Lyle double-bogeyed the same hole, Chalmers found himself two clear of Every and four ahead of Lyle.
Every never got back into first, but Lyle rebounded from the miscue at 11. He birdied 12 and 13, then added another at No. 16 to get in at 9-under par.
Chalmers bogeyed 14, 15 and 17, but birdied 16 and 18. That left the two tied for the lead with one round to go.
Chalmers has one Nationwide Tour victory, which came at the 2005 Boise Open. Lyle has yet to visit the winner's circle on the circuit.
Bubba Dickerson (65), Peter Tomasulo (67) and Todd Fisher (68) are knotted in fifth place at 5-under-par 208. Former Ryder Cupper Steve Pate shot an even-par 71 and is alone in eighth at minus-4.
Josh Broadaway (68), Dicky Pride (68) and Michael Sim (69) are tied for ninth at 3-under 210.