The world No 1 hauled himself to within a shot of the lead after back-to-back birdies at 12 and 13 to move to four under for the day, but his all-round game deserted him as he bogeyed the next three holes.
McIlroy managed to stop the rot with two closing pars for a one-under 71, but he will go into Sunday’s final round seven strokes adrift of in-form leader and his Ryder Cup team-mate Henrik Stenson.
The 25-year-old appeared to be at odds with his swing from the moment he tugged his opening tee shot into the trees, although he did well to scramble five pars before two-putting from 12 feet for a birdie at the long sixth.
McIlroy then chipped in from a tricky lie in the rough over the back of the ninth green to get to 10 under, and he looked in good shape to set a testing clubhouse target when he followed a well-crafted birdie at the long 12th with a 15-foot putt for another at 13.
But he could not get up-and-down after coming up short with his tee-shot to the short 14th, and he three-putted the next before following a fluffed chip at 16 with another three-putt to run up an ugly six.
McIlroy now hopes to pile pressure on the leaders with a fast start to his final round, but he admitted he faces a huge task to mark his Bay Hill debut with a victory in his last competitive outing before the Masters.
“I’m frustrated,” he told Sky Sports 4. I birdied 12 and 13 and got right into the thick of things and then to give three shots back in quick succession was not really the plan.
“I’m disappointed with the way I finished but I played well for the most part.??I need to try to get off to a good start tomorrow, shoot a good score and see??where that leaves me.
“There’s a lot of quality players above me. I would need to get off to a very??fast start (to win). 30 or 31 on the front nine it’s possible; just try to start??better and go from there.”