Slattery carded three birdies and made par on every other hole at the El Encin Golf Hotel for a 14-under 202 for the tournament.
Rumford had a 68 for the round and Gagli shot 69 in a rollercoaster round that included four bogeys and an eagle on the par-5 fourth hole.
Luke Donald, the defending champion and current world No. 1, finished with a 2-under 70 after bogeys on three of the last five holes. He was at 8-under for the event and tied for 10th place.