In the first accident, a double-decker tour bus carrying people from Bangkok to a merit-making function in Kanchanaburi hit a pickup truck and overturned as it was negotiating a curve on Saeng Chuto Road in Tha Maka district in Kanchanaburi at about 9.30am.
Two female passengers, identified later as Jinda Yukhongpan, 62, and Wipanrat Jongnarangsin, 61, were killed at the scene.
One died later in hospital. Thirty nine other passengers were injured.
Police said all passengers on board the tour bus, operated by Heng Heng Tour Co, were mostly vendors from Wongwian Yai, Ban Khaek and Khlong San markets.
The group had chartered the bus to attend a merit-making function at Wat Wang Khanaithayikaram in Kanchanaburi's Tha Muang district.
Police said the bus left Bangkok early in the morning and they suspect the driver might have fallen asleep at the wheel, leading to the accident.
In the second accident, four people were killed and at least 50 others were injured when a passenger bus overturned in Chiang Mai province Thursday morning.
The accident took place between kilometre markers 25 and 26 on Chiang Mai-Mae Hong Son Road in Chiang Mai's Hot district at 9.40am.
The bus, which was carrying passengers from Mae Sariang bus terminal in Mae Hong Son to Chiang Mai, tried to avoid hitting a motorcycle, causing it to leave the road and overturn, killing four of the passengers, said police.
Meanwhile, the number of road-related deaths during the Songkran holiday has so far reached 277, with 2,926 people injured. Deaths are down, but injuries up.
From April 11 to April 16, there were a total of 2,754 road accidents across the country, 173 more than in the same period last year, said caretaker Deputy Interior Minister Wisarn Techathirawat, citing figures he had obtained from the centre for road accident reduction.
The number of deaths was lower than last year by nine, while the number of people injured was higher than last year by 143.
On April 16 alone, there were 273 road accidents nationwide that claimed 29 lives and injured 283 others, said Mr Wisarn.
Drink driving was the major cause of road accidents.
Seven provinces — Amnat Charoen, Chai Nat, Phetchaburi, Lop Buri, Ang Thong, Phangnga and Yala — reported no Songkran-related fatalities, he added.
Chiang Mai reported the highest number of road accidents over the six-day period, at 107, while Nakhon Ratchasima had the highest number of road-related deaths, with 13.