Bjorn, who matched the course record with a 6-under 66 in Friday's second round, posted a 5-under 67 on Saturday. Casey shot a 4-under 68 and Wall birdied the last in the final threesome to card a 2-under 70. The trio is knotted at 5-under-par 211.
The tournament finally caught up on Saturday after a six-hour wind delay wreaked havoc on the event Thursday. The second round was completed Saturday morning, the 36-hole cut was made and all 72 players who made the cut finished the third round on Saturday.
There is horrible weather in the forecast for Sunday, so the players will go out in threesomes off both the first and 10th tees. Times were moved up as the inclement weather is scheduled for the afternoon.
Darren Clarke shot a 5-under 67 on Saturday and is tied for fourth place with Ross Fisher (69) and Peter Hedblom (70). This trio is only a shot back at minus-4.
Bjorn, who seemed out of the tournament with an opening round of 6-over 78, parred his first hole in the third round, then broke into red figures with a tap-in birdie at the second. The Dane made it two in a row at the 220-yard, par-3 third when his 3-iron tee ball stopped 2 feet from the flagstick.
He parred his next 11 holes before he came to the par-5 15th. Bjorn missed the green with a 3-wood, but chipped to 2 feet. He converted the birdie putt to reach 3 under par for the championship.
At the par-5 closing hole, Bjorn found the putting surface with his 3-wood. The ball rolled 40 feet past the cup, but Bjorn drained the long eagle putt to stake his claim to the lead.
'Today was even better than yesterday,' admitted Bjorn, who tied the record that was established in last year's event on Friday. 'It was very, very good. Yesterday I holed a lot of good putts. Today I holed one putt and that was on the last, which leaves me in a very good position for tomorrow.'
Bjorn is the most experienced of the co-leaders with eight European Tour victories and two Ryder Cup appearances. His last win came at last year's Dunlop Masters.
Casey, who played in his only Ryder Cup two years ago at Oakland Hills, flew out of the gate on Saturday. He rolled in a 15-foot birdie putt at the first, then collected back-to-back birdies with a 3-footer at the second. Casey blasted his third out of a bunker to 12 feet at the par-5 fourth and ran home the birdie try.
He reached five-under par with a 15-foot birdie putt at the driveable, par-4 13th. Casey fell out of the lead at 16 when his approach missed the green left and his chip stopped 15 feet short of the hole.
The Englishman got back into a share of the top spot at the last when he two- putted from 35 feet.
Casey has five European Tour titles, including a win at the first Volvo China Open on the 2006 schedule. He was in a similar position last week at the British Masters when he owned a two-shot lead with 18 to play. Casey ended up sharing fifth place, but learned some lessons.
'I try and learn something from every round or every tournament,' acknowledged Casey. 'I'm pleasantly surprised to be back up there. I feel like I've hit the ball nicely so far this week.'
Wall, whose only tour win came at the 2000 Alfred Dunhill Championship, was 2 over par through eight holes, but rebounded with a birdie at the par-4 ninth.
He got back into the tournament with back-to-back birdies from the 12th. That put him at 4 under par for the championship, but he squandered some chances coming into the clubhouse.
Wall missed a 15-footer for birdie at the 16th, then his 25-foot birdie putt at 17 died right of the hole at the very end. He elected to hit an iron off the tee at the closing hole, but the decision paid off with a 25-foot birdie putt to join Bjorn and Casey in first.
Jarrod Lyle is alone in seventh place after a third-round, 1-under 71. He completed 54 holes at 3-under-par 213.
Padraig Harrington (69), second-round leader Nicolas Colsaerts (74), Robert- Jan Derksen (68), Robert Karlsson (72), Ian Poulter (73), Tom Whitehouse (72) and 1999 British Open champion Paul Lawrie (71) are knotted in eighth place at minus-2.