Ilonen, whose two European Tour wins came in 2007, briefly moved two clear of the field with birdies at the 14th and 15th, the latter from six feet after a superb approach to the 489 yard par four.
But a first bogey of the day on the 16th dropped him back to four under and one ahead of McIlroy, who had birdied the par five 12th after hitting his second shot just through the green and chipping to three feet.
Rose was a shot further back after a clumsy chip on the 13th cost him a bogey five.
Ilonen dropped a shot on the 16th after a pulled drive but birdied the 18th to card a five under par 67 that might prove a difficult target to match given the conditions.
"It was pretty nippy when we started and stayed that way," Ilonen said. "All day I was thinking 'Can I get these waterproof trousers off?' and never managed it.
"We even had hail on the ninth tee. I said to a friend last night it was going to snow today and she laughed at me. This is the British summer."
Italy's Matteo Manassero had become Ilonen's nearest challenger on four under with three to play, with Rose bouncing back from his bogey on the 13th with a birdie on the 14th, a hole McIlroy three-putted in the group behind.
Best start of the day had come from regional qualifier Jason Levermore, who fired a hole-in-one on the second with an eight iron from 155 yards.
|T4||JIMÉNEZ, Miguel Angel||ESP||18||-9|