"What we know at this time is that the alleged offender was known to one of the victims," Waikato West area commander Inspector Will Loughrin said.
"This was an incredibly traumatic event for those either dining or working at the restaurant and we'll ensure they receive any support required," Loughrin said.
"A scene examination will continue today and the community will notice an increased police presence in the area as we continue with our enquiries."
Emergency services were called to the restaurant on Campbell St, in Leamington, following a report of a grievous assault about 8.45pm on Wednesday, police said earlier.
"Upon arrival, police located three people with serious stab-related injuries," said Detective Sergeant Anthony Hodgson.
Campbell St was closed between Burns St and Shakespeare St overnight while a scene examination took place.
Anyone who has any information in relation to the incident is encouraged to get in touch with Police via 105 and quote event number P050463060.