Ben Mason fired a 5-under 66 to move into a share of second place. He was joined by Garry Houston at 8-under-par 205.
Heavy rain and lightning delayed the third round for nearly four hours early on Saturday. When Rumford finally took to the Val Course at Aa Saint Omer Golf Club, he wasted little time and picked up his first birdie of the day at the par-4 first.
Rumford collected three additional birdies on his front nine to jump to 11-under. The Australian then added a birdie at the 12th and after a bogey by Mason at the 15th, Rumford stood five shots clear of the field.
He cooled off down the stretch and found trouble with a bogey at the par-3 17th. Rumford parred the closing hole to secure a three-shot advantage heading into the final round.
'The three-putt on the 17th was the only blemish of the day,' said the 25- year-old. 'I hit a great shot to 10 feet at the last which I would have been very pleased to hole but it stayed out. Still I'm quite happy with the day. I would have liked to be two or three further ahead but just focus on tomorrow's round. It is the final round, just keep playing how I have been playing and see how I go.'
Rumford, who is looking to regain his European Tour card, can earn his first career victory on the circuit in wire-to-wire fashion.
Mason got things going early with back-to-back birdies to start his round. He added a pair of birdies starting at the par-4 sixth to move to minus-7.
The Englishman then birdied the 12th but stumbled with a bogey at the par-4 15th. He responded with a birdie at the 17th to finish in a tie for second.
'Tomorrow I'll just go out there and try and enjoy myself,' said Mason. 'It is the first time I have played in the last group of a European Tour event.'
Houston tallied two birdies and one bogey over his first nine holes to make the turn at 6-under. The Welshman notched two more birdies on his inward half to join Mason at 8-under par.
'It's been a while since I was contending for the lead so it is exciting to be up there,' said Houston. 'I feel I am playing better than ever and thinking properly on the golf course so I'm looking forward to tomorrow.'
The Swedish duo of Mattias Eliasson and Peter Hanson joined Italy's Federico Bisazza in a tie for fourth at 5-under-par 208.
Another Swede in Martin Erlandsson was one shot further back at 4-under-par 209 along with Alvaro Salto of Spain in a tie for seventh.
Sebastien Branger, the low Frenchman, struggled with a 1-over 72 to finish at 3-under-par 210 in a tie for ninth with England's Stuart Little and Bjorn Pettersson of Sweden.