Westwood made it four birdies in a row with a ten foot putt on the ninth and so turned in 31, one behind McIlroy.
Donaldson joined Westwood on 11 under, but then three-putted the ninth for a bogey six and slipped back alongside last week's winner Thomas Björn, Boyd, Danny Willett and World Number Five Martin Kaymer.
Westwood became joint-leader again with a two-putt birdie at the long 14th and McIlroy did well to stay alongside him, getting up and down from a bunker on the 13th.
They were a shot in front of Dane Søren Hansen, who came into the picture with an 18 foot eagle at the 15th.
The chances of a last-day showdown between Westwood and McIlroy increased when the former made a great up and down over a bunker to birdie the 15th and the latter chipped to six feet on the 14th.
At 13 under they were two clear of Donaldson, Hansen and his fellow Dane Søren Kjeldsen.
McIlroy went one ahead again with a two-putt birdie at the 15th, but then three-putted the next and bogeyed the 17th as well after going over the green from a fairway bunker.
Donaldson, meanwhile, had a hat-trick of birdies from the 13th and joined Westwood - round in a best-of-the-day 64 - on 13 under, one in front of McIlroy.
|T3||VAN ZYL, Jaco||RSA||18||-15|
|T6||ANDERSSON HED, Fredrik||SWE||18||-14|