Harmison, 29, who made the last of his 57 Test appearances in March, is included in the 13-man squad along with Paul Collingwood and a fit-again Ryan Sidebottom.
Darren Pattinson and Chris Tremlett, the stand-by bowlers in the second Test which South Africa won by 10 wickets at Headingly last week, have not been selected.
Harmison, who was dropped after England's winter tour of New Zealand, has regained his place after showing some excellent form for Durham in the County Championship this season.
He is the leading wicket-taker in Division One with 41 wickets in nine first class games with an average of 22.82 this season.
Chairman of selectors Geoff Miller told Sky Sports News Harmison had been added to the team to improve the attack which has taken just 13 wickets against South Africa in the series so far.
"Steve has bowled well this year for Durham and deserves his place in the side and his inclusion in the squad gives us another option in terms of our bowling attack for Edgbaston," he said.
"I hope it represents a rebirth for Steve, the ball is in his court."
South Africa go into Wednesday's test 1-0 up in the four-Test series.