The explosions occurred almost at the same time at around 10pm. The first bomb was detonated at a petrol station at Nong Hoi subdistrict in the provincial seat of Chiang Mai.
Four people--three petrol station employees and one customer--were wounded and sent to a nearby hospital.
The investigators found the trigger of the bomb and guessed it to be an RGD5 hand grenade. Police were checking CCTV footage to seek evidence and the bomber's identity.
The petrol station was operated by a company of a former Uttaradit Senator who was reportedly supporting the anti-government People's Democratic Reform Committee (PDRC).
The second bomb was exploded at the parking lot of Andaman Seafood restaurant on Mahidol Road of the provincial seat.
Police said, according to the initial investigation, the bomb was a homemade gunpowder explosive device. The power of the blast damaged several cars, but there were no reports of injuries.
The third blast has occurred at office of Chiangmai Beverage Co, an affiliate of Boonrawd Brewery Co in Saraphi district. There were no initial reports of injuries.
According to an initial investigation, two M-79 grenades were shot at the office, one failed to explode, while the another one exploded.
Earlier at around 9.30pm in the eastern resort province of Chon Buri, two M-79 grenades landed near a PDRC stage in the provincial seat.
Kantapon Sukhumalin, a leader of the PDRC in Chon Buri, said some 2,000 protesters had rallied at the Bang Sai subdistrict while attackers in a pickup truck parked and fired two grenades at them.
One grenade landed in front of Bang Sai municipality office, damaging glass windows, a pickup truck, and a car. Another grenade caused damage to Bang Sai knowledge and development centre.