Fernandez-Castano began the day one shot behind joint leaders Luke Guthrie and Rafael Cabrera-Bello but picked up shots at the seventh, eighth and 11th to improve to 10 under par.
Molinari was six shots off the pace at the start of the day and despite a front nine of 34, his chances looked to have disappeared with a bogey on the 12th.
However, the Ryder Cup star then went on a brilliant scoring run, making an eagle on the par-five 13th and following that with four birdies in succession to join Fernandez-Castano on 10 under.
Defending champion Peter Hanson had set the clubhouse target on seven under par after a brilliant closing 63, the Swede having started the tournament - which does not have a halfway cut - with a round of 79.
"I was playing with Paul McGinley, the Ryder Cup captain, and I guess I wanted to impress him," Hanson joked after a round containing 11 birdies and two bogeys.
"I had no expectations on coming back from injury and I was obviously very rusty on Thursday. But I have felt better and better throughout the week and to finish with a round like that is very pleasing. As a defence, it wasn't bad!"