This win helps England take an unassailable 3-0 lead in the five-match NatWest women's series. The hosts earlier recorded victories at Canterbury and Lord's.
England, riding on Lydia Greenway's steady 61*, posting a respectful 185 all out in 47.3 overs. Ejclerc Rainford-Brent made a strokeful 45 inclusive of nine boundary hits. Sarah Taylor chipped in with quick 23. van der Westhuizen was the pick of South African bowlers with 4/30 in her 10 overs. APC Kilowan claimed two wickets.
South Africa eves chasing a modest total faltered from the word go and eventually were bundled out for 108 in 42.2 overs. Anne Smith stood tall amidst the SA batting ruins with 57-ball 30.
Right-arm medium fast bowler Laura Marsh was the wrecker-in-chief for England with 4/19 in 9.2 overs. Holly Colvin plucked three vital wickets conceding just 20 runs in her quota of 10 overs which included three maidens. Jenny Gunn returned with 2/23.
England after winning the toss and electing to bat first, got off to a poor start losing Caroline Atkins in the third over of the innings. Lord's centurion chipped a simple catch to Charlize van der Westhaizen.
Fellow opener Ebony Rainford-Brent, recalled to the side, looked in good touch and was particularly aggressive against anything pitched short.
Claire Taylor looked in ominous form at the other end, a sublime cover drive in the seventh over evidence of her class, but, seemingly on the cusp of a big score, she departed in freakish circumstances.
After failing to connect with an extravagant drive off Ashlyn Kilowan, Taylor was stumped by an alert Trisha Chetty whose under-arm throw caught the star batter short of her ground.
England stuttered to 61 for three soon after when Edwards mistimed a rank long-hop from van der Westhaizen straight to Sunette Loubser at short midwicket. The hosts lost a heap of wickets in the middle overs and slumped further to 136/8.
Greenway could only stand at the other end and watch as the wickets continued to tumble. The resolute Greenway, who made her 50th one-day appearance in the 225-win at Lord's, used all her experience to dig England out of a hole.
Picking off singles as she found a useful ally in Holly Colvin, Greenway held the innings together and ensured England used the bulk of their overs when it looked like lunch might be served early.
England seamers made early inroads thro' Brunt and Gunn. Brunt trapped Claire Terblanche lbw with just four scored.
Gunn then got into the wicket taking act removing Brits and Smith to leave visitors in trouble at 53/3. Laura Marsh and Colvin then ran amok as SA eves could manage to score 108 all out in 42.2 overs.
The two teams will stay back at Shenley for the fourth ODI on Thursday.
England eves 185 all out in 47.3 overs (Lydia Greenway 61 n o, Ejclrc Rainford-Brent 45, Sarah Taylor 23; van der Westhuizen 4/30, Kilowan 2/34) beat South Africa 108 all out in 42.2 overs (Anne Smith 30, S Loubser 22; Laura Marsh 4/19, Holly Colvin 3/20, Jenny Gunn 2/23).