All three have been vaccinated.
King, an independent who caucuses with the Democrats, said in a statement that he was tested Thursday morning after he "began feeling mildly feverish" on Wednesday.
"While I am not feeling great, I'm definitely feeling much better than I would have without the vaccine," King said. "I am taking this diagnosis very seriously, quarantining myself at home and telling the few people I've been in contact with to get tested in order to limit any further spread.
His announcement came just hours after Wicker, a Republican, also tested positive for Covid-19 on Thursday morning, according to a statement from his office.
"Senator Wicker tested positive this morning for the COVID-19 virus after immediately seeking a test due to mild symptoms," Wicker spokesman Phillip Waller said in the statement. "Senator Wicker is fully vaccinated against COVID-19, is in good health, and is being treated by his Tupelo-based physician. He is isolating, and everyone with whom Senator Wicker has come in close contact recently has been notified."
Hickenlooper, a Democrat, announced his diagnosis later in the afternoon on Thursday.
"After experiencing mild symptoms, I tested positive for a breakthrough case of COVID-19," he said in a statement.
"I'm feeling much better and will continue to isolate at the direction of the Congressional Attending Physician. I'm grateful for the vaccine for limiting my symptoms and allowing us to continue our work for Colorado. If you haven't been vaccinated, don't wait for the virus — get the shot today, and a booster when it's available too!," he said.