On a day when only three players in the 80-strong field broke par, as strong winds sent scores soaring, Wallace had to birdie the 18th to card a three-over-par 73.
That was 11 shots worse than his second round of 62 but good enough to reach 11 under par and stay four clear of a chasing pack headed by Spain's Alfredo Garcia Heredia.
England's Andrew Johnston and Sweden's Henrik Norlander were a shot further back, with Edoardo Molinari's superb 69 giving him a share of fifth place alongside Alex Fitzpatrick, Scrivener and home favourite Cedric Gugler.
Scrivener, who had been one shot off the lead on the first day, is also five under in a share of fifth after a creditable 72.
"Where do I start?" Wallace told Sky Sports when asked to assess the difficulty of the conditions. "Brutal, hard. Really hard.
"I hit the ball I felt like just as good, hit my areas - they were the wrong areas by the looks of it. Some holes like 17 were potentially impossible.
"I said the other day it was cold and windy, but that (today) was crazy, that was mental. The greens were fantastic but just really fast as well."
Asked if he was pleased at mentally handling the situation better than he might have previously, Wallace added: "Massively. I'm really, really proud.
"I'd have lost my head easily there from the get go. A missed (birdie) putt on the first and then a bogey on the second - it's obviously easier when you have some sort of lead and it's playing really hard but I just tried to treat it like a major where every hole has its own merit.
"Today wasn't really about trying to shoot under par, it was about trying to keep my lead and I did that and I'm going to try and take the ego out of the three over."
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