The leaders are 10-under for the tournament, but Oosthuizen squandered a great opportunity to join them when he pulled his approach to the last into the sand.
He got to within 18 feet of the pin, but missed his putt and that dropped the 27-year-old to nine-under.
Johnson matched Oosthuizen’s two-under 70, while halfway leader Choi managed a 71.
“It was a bit disappointing,” Oosthuizen said of his closing five. “I made two thinking errors on the course and it cost me two bogeys.”
Choi, whose only European success was the 2003 German Masters, endured a trying round in the wet and windy conditions.
The 40-year-old hit his second to the long 13th into the edge of the lake and chose to take his shoes and socks off and wade in to try to splash the ball out. He went over the green with it, before salvaging his par, but found himself in a three-way tie when Johnson and Oosthuizen birdied the 622-yard 15th and he missed from five feet.
New Zealander Mark Brown, with a 67, and Argentina’s Rafa Echenique (72) are fourth on seven-under.
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