Nilsson, who is winless on the European Tour, completed 54 holes at 11-under- par 202, which is two off the 54-hole scoring record.
'Today's round was very satisfying, especially on the front nine. I got off to a good rhythm from the start, and just carried it on,' Nilsson said. 'The key was probably my putting. I've putted well all week really, but today nearly everything I hit dropped in the hole.'
James Morrison managed a 1-under 70 in the third round and remained alone in second place at 7-under-par 206.
Second-round leader Wade Ormsby stumbled to a 3-over 74, which dropped him into a share of third place at 4-under-par 209. He was joined there by first-round leader Fredrik Andersson Hed (71), Sion E. Bebb (67) and Jose- Filipe Lima (69).
Nilsson, who is the 478th ranked player in the world, opened with a birdie on the first at Aa Saint Omer Golf Club. After three straight pars, he birdied the fifth and made it two in a row on No. 6. Nilsson has birdied the sixth all three days.
The Swede made eagle on the par-5 ninth to make the turn at minus-10, where he was well in control of the lead.
Nilsson, who held the 54-hole lead last year, traded a birdie for a bogey from the 11th. He parred five straight from the 13th before running home his final birdie effort on the par-3 17th.
'I think I've led once before going into the final day, but never by this much,' admitted Nilsson, who finished second at this event last year. 'So it's going to be a tough day, but I'm also looking forward to it.'
Morrison bogeyed the first and eventually got that shot back as he birdied the seventh for the third consecutive day. Around the turn, he dropped another stroke on the 10th, but atoned for that error with a birdie on the 12th.
The 24-year-old faltered to another bogey on 15, but closed with birdies on 16 and 18 to finish under par for his round and four strokes off the lead.
Simon Wakefield (70), Matthew Millar (70), Lorenzo Gagli (68), Adam Gee (71) and Ake Nilsson (68) share seventh place at 3-under-par 210.