Donaldson was level with Denmark's Jeppe Huldahl on six under par.
The 33-year-old Macclesfield professional stood on five-under-par with Manchester-based pal Simon Dyson overnight.
But Thongchai Jaidee of Thailand fired five birdies on the front nine to move to the top of the leaderboard during the third round.
The 39-year-old, who returned to action this week after missing the Johnnie Walker Classic due to illness, began his round on four under, two shots off the lead, after carding a 69 yesterday.
Second round leader Steve Webster was one of three players in a tie for second place after picking up a birdie on the fifth to improve to seven under after eight holes.
He was joined on that mark by Scotland's Scott Drummond, who was three under for the day through eight holes and Alexander Noren of Sweden, who was five under through 13 after making five birdies on his first 10 holes.
Jyoti Randhawa of India held the clubhouse lead on six under after shooting a six-under-par 66 that included an eagle and six birdies against two bogeys.
MANCHESTER'S Andrew Murray trailed three shots behind compatriot Bob Cameron in ninth spot when the new European Senior Tour season swung into action with the Aberdeen Brunei Senior Masters.
Murray offset two bogeys with a trio of birdies for a one-under-par 70.
BOLTON'S Steve Parry fired a one-under-par 71 to share top place with former Marriott Worsley Park amateur Marcus Armitage and Welshman Matthew Tottey in the final Players' Tour winter pro-am at Caldy.
Armitage, who has moved to Yorkshire club Howley Hall, also won a clothing deal for leading the series.
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