Police responded at 3:49 a.m. Sunday to a report of injured people at 162 S. Cherry St.
Police accuse Nathan L. Kropp, 30, of cutting his neck with a razor blade "after hearing voices in his head" and then cutting the necks of his girlfriend and their 4-year-old child.
The three were taken to Mercy Hospital and Trauma Center in Janesville, where Kropp remained under police guard.
Kropp is being held on a tentative charge of recklessly endangering safety with a domestic-violence enhancer.
No charges had been filed as of Monday because of a pending mental evaluation, Janesville police Lt. Timothy Hiers said.
A psychiatric evaluation will help determine where Kropp goes after his release from the hospital, police said.
No new information was available Monday about the conditions of Kropp's girlfriend and the child.
A nursing supervisor on Monday would not report Kropp's condition and said only hospital administration could do so.