MOSCOW, 5 Sept 2015, (AFP) - After play had been delayed by 95 minutes in the morning due to heavy rain, Slattery, who had been in second place overnight, carded a four under par 67 to move to 13 under.
Scotland's Craig Lee and Estanislao Goya of Argentina were tied for second place on 11 under, with defending champion David Horsey a further stroke back.
Horsey is looking for back-to-back wins, having triumphed in Denmark last week.
Slattery made gains at the third and fifth holes to take a share of the lead at the turn. A birdie at the tenth handed him the outright lead but he double-bogeyed the next to slip back to ten under.
The 37-year-old recovered admirably, carding birdies at the 12th and 14th before adding another at the last to finish the day with a healthy lead.
Slattery said he was looking forward to the final round in what is the first tournament awarding points for next year's Ryder Cup.
"It's very similar to my last win actually, I had a two shot lead there on the final day.
"I remember losing the lead quite early, but it pushed me on to play well again so it's similar, similar feelings, and I'm just going to go out and enjoy it tomorrow.
"It's the old cliché, take it one shot at a time -- that's what I'll be doing tomorrow and I'm looking forward to it."
Lee carded five birdies on his way to adding a 66 to the rounds of 67 and 69 he shot on Thursday and Friday respectively.
Goya fired four birdies as he posted a flawless 67, while Horsey mixed six birdies with a bogey to revive his challenge.
Overnight leader Bradley Dredge of Wales slipped to nine under par after firing three birdies and four bogeys in his one over par 72.
Collated third round scores and totals in the European Tour's Russian Open at the Skolkovo Golf Club, Moscow on Saturday:
(GBR and IRL unless stated, par 71)
200 - Lee Slattery 66 67 67
202 - Craig Lee 67 69 66, Estanislao Goya (ARG) 68 67 67
203 - David Horsey 67 70 66
204 - Oskar Henningsson (SWE) 69 68 67, Bradley Dredge 66 66 72
205 - Pablo Martin Benavides (ESP) 68 67 70
206 - Ben Evans 67 68 71, Jason Scrivener (AUS) 70 66 70, David Drysdale 68 69 69, Danie Van Tonder (RSA) 69 69 68, Matthew Southgate 68 69 69
207 - Daniel Vancsik (ARG) 69 68 70, James Heath 70 66 71, Lasse Jensen (DEN) 70 69 68, Andrea Pavan (ITA) 70 69 68, Keith Horne (RSA) 67 70 70, Scott Jamieson 65 71 71
208 - Daniel Gaunt (AUS) 65 69 74, Maximilian Kieffer (GER) 68 69 71, Simon Griffiths 68 71 69, Thitiphun Chuayprakong (THA) 67 71 70, Dodge Kemmer (USA) 71 69 68, Jerome Lando Casanova (FRA) 69 68 71, Chris Doak 71 67 70, Michael Hoey 71 66 71