The win gave England an incontrovertible 3-0 triumph in the series, with the 4th and last ODI to be played on the same ground on Tuesday.
Kevin Pietersen hit his eighth ODI hundred while Alastair Cook missed a 3rd successive century.
The 31 year old Pietersen notched an unbeaten 98-ball 111 while opening partner Cook punch 80 to assist their team repair a modest 223-run target set by Pakistan with 12.4 overs to spare.
Pietersen, whose last hundred came against India at Cuttack in November 2008, hit his 10th boundary to bring up the win.
He pushed paceman Aizaz Cheema towards square-leg for two to reach his hundred off 93 balls, his first in 36 innings.
Cook, who hit a career-top 137 and then 102 in the first two matches, was shaping well for his 3rd century but fell when he was caught behind off spinner Saeed Ajmal after a fluent 98-ball knock, hitting six boundaries and a six.
Cook fell with 53 still needed before Eoin Morgan (24 not out) helped Pietersen see off the target.