The trio sit on 7-under-par overall, one shot ahead of German golfer Max Kieffer.
Pieters, who tied for fourth at the Masters in his last tournament last month, recorded a round of 3-under 69 to set the target for the late starters.
Molinari dropped two shots on the opening nine but roared back into contention with three birdies in the closing six holes, including at 17 and 18, for a 70.
Jamieson dropped three shots in his first three holes to fall five off the lead, but a run of six birdies in seven holes around the turn saw him surge back up the leaderboard. He carded 70, too.
Olympic champion Justin Rose needed an eagle on the final hole to make the cut. Rose sat on 2 over.
The Englishman, eight shots off the halfway lead, said: ''I had really good opportunities to birdie all the way back into the clubhouse and I didn't do it.''