The victims were named as Tate Myre, 16, Madisyn Baldwin, 17, and Hana St Juliana, 14. A fourth student - Justin Shilling, 17 - died on Wednesday.
Police allege the suspect, a pupil at the school, used a handgun that his father had bought on Friday.
No motive has yet been identified. A police update is expected on Wednesday.
Students have described hiding under desks during the attack.
Some students had reportedly stayed at home on Tuesday because of safety concerns.
