HILVERSUM, Netherlands, Sept 8, 2012 (AFP) - The big Englishman stretched his lead to five at one point with birdies at the seventh, 10th and 12th and it looked briefly like the rest were playing for second place.
But he dropped his first shot of the day on the 13th and a a wayward drive at the next led to another bogey as he had to settle for a one-under 69.
Larrazabal meanwhile birdied five of his last six holes en route to a 64 - the equal best round of the day - while Jamieson and Fernandez-Castaño each picked up shots on 17 and 18 to shoot 66s.
Peter Hanson, part of Europe's Ryder Cup Team and another player to birdie the final two, was one off the pace at 11 under, with an absorbing final day in prospect.
Storm, 114th on The Race to Dubai, attempted to remain positive, despite losing his healthy lead.
"I had two bad swings on 13 and 14," he said. "I had the tournament in the palm of my hand before that, but it should be a good day tomorrow. I've only had two bogeys in 54 holes, which is quite impressive.
"It's difficult leading like I was. I have a lot of things on my mind at the moment with keeping my card, so I'm just looking at getting the best result I can.
"Tomorrow, keeping my (European Tour) card will be in the back of my mind, but I have given myself the chance to win and if I am in there with six holes to go I will go for it."
Larrazabal was delighted with his late charge.
"I played great, on the front nine as well as the back," said the Spaniard.
"I will attack whatever I can attack tomorrow. I've not been in contention for a while, so I am going to have some fun and try to compete for the title."
Scotland's Richie Ramsay and Henrik Stenson of Sweden matched Larrazabal's 64 to climb into contention.
Ramsay, seeking back to back wins after his victory in last week's European Masters, and Danny Willett ended the day nine under, with Stenson one worse off alongside Nicolas Colsaerts, who carded a 67.
Leading third round scores in the European Tour KLM Open at Hilversum on Saturday (GBR and IRL unless stated, par 70):
198 - Gonzalo Fdez-Castano (ESP) 67 65 66, Graeme Storm 63 66 69, Scott Jamieson 68 64 66, Pablo Larrazabal (ESP) 69 65 64
199 - Peter Hanson (SWE) 66 66 67
201 - Richie Ramsay 71 66 64, Danny Willett 66 69 66
202 - Nicolas Colsaerts (BEL) 70 65 67, Henrik Stenson (SWE) 68 70 64
203 - Simon Thornton 71 64 68, Lorenzo Gagli (ITA) 69 67 67, Marcus Fraser (AUS) 69 68 66
204 - Shiv Kapur (IND) 67 67 70, Martin Kaymer (GER) 65 71 68, Bernd Wiesberger (AUT) 68 71 65, Richard Sterne (RSA) 70 68 66
205 - Garth Mulroy (RSA) 71 65 69, Anders Hansen (DEN) 70 67 68, Darren Fichardt (RSA) 70 67 68, Bradley Dredge 69 66 70
206 - Richard Bland 67 72 67, Jaco Van Zyl (RSA) 68 68 70, Raphael Jacquelin (FRA) 65 70 71, Felipe Aguilar (CHI) 66 74 66, David Drysdale 70 69 67, Richard S Johnson (SWE) 70 67 69, Marc Warren 69 71 66, Tommy Fleetwood 67 73 66, Richard Green (AUS) 69 69 68, Gregory Bourdy (FRA) 69 68 69