The Bangkok South Criminal Court approved warrants for the arrest of the seven suspects on Monday.
Four of them were then arrested in Sai Mai area yesterday. They are Taweechai Wichakham, 39, Sunthorn Phiphuannok, 49, Chatchawal Prabbamrung, 45, and Somsri Marit, 40, police chiefs said at a press conference on Tuesday.
The three others, still at large, are Suksant Lomwong, 32, Kannikar Wongtua, 38, and Wichian Sukpirom, 45.
When police arrested the four they also seized one M79 grenade launcher, 25 M79 grenades, one M16 rifle, 70 rounds of M16 ammunition, another rifle, five hand grenades and three pick-up trucks.
The four were charged on eight counts including premeditated murder, attempted murder and illegal posession of firearms and ammunition.
Pol Gen Somyos Phumphanmuang, a deputy police chief, said the four confessed to the charges. They said they planned the attack at a house in Sai Mai area. On Feb 23, they travelled together in the three pick-up trucks to the attack scene.
Mr Taweechai, one of the four, said he was in the second vehicle and he fired an M79 round from the bridge over the Pratunam intersection, targeting the PDRC protest site at the Ratchaprasong intersection.
The M79 round landed in front of the Big C store on Ratchadamri road, killing three people out shopping, including two children, and wounding 21 others.
The four suspects were then taken to the scene of the attack for a re-enactment of the crime.
Pol Gen Watcharapol Prasarnratchakij, the acting national police chief, was also at the press conference.