The Dutchman won the inaugural event in 2018 and a bogey free 67 on day three moved him to 11 under but he had plenty of company at the top of the leaderboard.
England's Callum Shinkwin bogeyed the last in his 66 to drop out of the solo lead, with South African Brandon Stone carding a 67 and Danish teenager Rasmus Højgaard signing for a 70 to complete the leading sextet.
Korhonen has two victories to his name, while Højgaard became the third youngest winner in European Tour history when he lifted the trophy at the AfrAsia Bank Mauritus Open earlier this season.
Shinkwin is looking for his first title after bogeying the last and losing in a play-off at the 2017 Scottish Open, and Valimaki is playing just his sixth European Tour event after coming through the Qualifying School.
Overnight leaders Højgaard and Stephen Gallacher were the early pacesetters and Gallacher led at the turn after birdies on the second and fifth but Shinkwin made a big move through the field.
The 26-year-old took advantage of the par five third and then put an approach to five feet at the sixth, capitalised on another par five at the seventh and holed a 20 footer on the eighth.
A two putt birdie on the 12th kickstarted another hat-trick of gains as he holed from ten and 15 feet at the 13th and 14th and led alone, before a poor second at the last brought a closing bogey.
That dropped him into a five way tie for the lead, with Valimaki and Korhonen already in the clubhouse.
The 21-year-old turned in 32 after birdies on the second, third, sixth and seventh and got up and down on the 12th before hitting a wonderful tee shot into the par three 13th.
He ran his first putt on the par five 16th into the fringe but made his birdie before a 12 foot left to righter on the 17th completed the lowest round of his European Tour career.
Korhonen flew out of the blocks with three birdies and after taking advantage of the seventh, he put an approach to four feet at the ninth.
A tap-in at the 15th had him in double figures and a stunning second into the tricky pin at the last put him in the clubhouse alongside his countryman.
Luiten birdied the first but then made nine pars in a row before birdieing the 11th and holing a very long putt at the 12th. His tee shot on the next almost brought a hole-in-one and he got up and down on the 16th to join the group at 11 under.
Playing partner Stone took advantage of the par five third, seventh and 16th, while also holing a long putt on the tenth and putting an approach to eight feet at the 14th.
Højgaard had birdied the third but was in trouble when he bogeyed the sixth and made a double on the seventh.
He hit back from ten feet at the eighth, 15 feet at the ninth and holed a long putt on the tenth to get those shots back but another bogey came on the 13th before he put an approach to eight feet on the next and made the most of the 16th.
Migliozzi started his round with three birdies and added further gains on the fifth and 12th before dropping a shot on the 15th.
Sordet carded a 66, one shot fewer than Saddier and Smith to sit a shot ahead of Finn Kalle Samooja and two clear of Gallacher - who came home in 40 - two time Major Champion Martin Kaymer, Belgian Nicolas Colsaerts and England's Robert Rock.