|South Africa 284 (Kapp 150, Cross 4-63) & 55-3 (Wong 2-8)|
|England 417-8 dec (Sciver 169*, Davidson-Richards 107)|
|South Africa trail by 78 runs|
Debutant Issy Wong took two late wickets to aid England's push for victory over South Africa on day three of the one-off Test at Taunton.
A three-hour rain delay looked to have ended the day's action, only for Wong to strike twice when the players returned for 12.5 overs from 18:30 BST.
Lara Goodall tickled down the leg side and Laura Wolvaardt was caught at gully to leave the Proteas 55-3, 78 behind.
England had earlier pushed their first innings to 417-8 declared, the fourth-highest total they have made in a women's Test.
Nat Sciver ended unbeaten on 169 not out, also fourth on the all-time list of individual scores by an England woman in Tests.
Leading by 133, England removed South Africa opener Andrie Steyn, before two lengthy rain breaks allowed only eight overs in nearly six hours.
The weather looked to have frustrated England, only for Wong's double strike to keep them on course for a first Test win since 2014.
Wong worth the wait
When the second heavy shower arrived at 15:40, there seemed little prospect of any more play in the day.
However, it is to the credit of the umpires and groundstaff that they persevered, and England were given a bonus session in the evening gloom.
Goodall could earlier have been caught behind on eight off Lauren Bell, but England failed to appeal.
When South Africa were asked to bat again late in the day, they had little to gain and much to lose, yet Goodall and Wolvaardt looked accomplished.
However, the extra pace of Wong presented a new threat, with Goodall unable to resist following a full ball down the leg side, flicking it through to wicketkeeper Amy Jones.
In Wong's next over, Wolvaardt poked at one that bounced, and Sciver took a good catch low to her right at gully.
With England crowding the bat, South Africa captain Sune Luus somehow survived to be four not out, with nightwatcher Tumi Sekhukhune on one.
There are 109 overs available on Thursday's fourth and final day for England to earn their first home Test victory in 17 years.
Sciver pushes on before rain arrives
Sciver's huge second-day partnership with Alice Davidson-Richards had taken England to 328-6, a lead of 44.
She resumed on 119 in the company of Sophie Ecclestone, the seventh-wicket pair initially respecting the South Africa bowling before pushing towards the declaration.
They added 86, Ecclestone registering her highest international score of 35 before she was lbw to left-arm spinner Nonkululeko Mlaba.
Kate Cross was lazily run out at the non-striker's end, failing to ground her bat before Nadine de Klerk's throw hit the stumps, prompting England's declaration.
It left Sciver undefeated and, with this effort added to the 148 not out she made in the World Cup final loss to Australia in April, the all-rounder has now scored 317 international runs since her last dismissal.
'England deserve to win' - reaction
England bowler Issy Wong on Sky Sports: "We've had a frustrating day, the weather hasn't got the memo, but we were really keen to use those overs to make an impact on the game and push it forward to hopefully get a result tomorrow.
"The night before the game Lisa Keightley sent me a text to say 'Are you free for a chat?' and I thought she was going to send me to home but said 'You're making your debut'.
"Fingers crossed the weather plays its part and we can really press for a result tomorrow."
Former England bowler Anya Shrubsole on Test Match Special: "England will be coming out to get seven wickets. One of South Africa's main batters is out and they will already be thinking about batting for a draw.
"England have been aggressive with the ball and put themselves in a really good position in this game. They deserve to win."
England all-rounder Georgia Elwiss: "England will be buzzing they've got those two wickets. Issy Wong has done exactly what Heather Knight wanted from her and she looks like this is what she is made for.
"But with the talent they've got in their line-up, there is no reason South Africa can't bat out the day tomorrow."