Open Championship winner Louis Oosthuizen remains in with a good chance of making history - the South African is one behind after dropping a shot at the last.
Oosthuizen, trying to become the first player ever to follow a Claret Jug victory with another European Tour win a week later, was tied with Johnson and Choi when he pulled his approach to the last into sand.
Splashing out to 18 feet and missing the putt dropped the 27 year old South African to nine under par.
Johnson matched Oosthuizen's two under 70, while halfway leader Choi managed a 71 after experiencing what he called "all four seasons in one day."
It was windy, it was wet and for one stroke it was very cold too for the 40 year old, whose only European success was the 2003 German Masters.
Choi 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.
Oosthuizen said after his closing five: "It was a bit disappointing.
"I made two thinking errors on the course and it cost me two bogeys."
The first led to a three-putt six on the long ninth.
He also drove into the water at the 13th, but after taking a penalty drop he hit his third to 15 feet and made the putt for one of four birdies.
"I was a bit gutted after the ninth and on the last it was a stupid second to that pin.
"I've got my goals set on winning again, though, and it will be right up there to me - a great achievement after winning last week."
Johnson has a friend - a local restauranteur, radio host and someone he describes as "kind of a comedian" - acting as his caddie this week.
His regular in the States, where the 33 year old is based, is Lance Ten Broeck, but he was playing in The Senior Open Championship presented by MasterCard at Carnoustie this week.
Johnson, like Choi, has one European title to his name, but that was eight years ago in Australia.
"I decided to play here about three weeks ago," he said. "I heard Canada was such a great course, but it kind of worked out to come here.
"It's very exciting because the crowd is really behind me. I was not nervous at all and I'm really looking forward to tomorrow.
"The wind was blowing really, really hard and I was not driving it great. I had to rely on my short game and it came through again."
New Zealander Mark Brown, with a best of the day 67, and Argentina's Rafa Echenique (72) are joint fourth on seven under, while Italian Edoardo Molinari, who could force himself into the all-important top nine on the European Ryder Cup table with a top two finish, is one further back.
|1||JOHNSON, Richard S||SWE||18||-11|