Six-month-old Himanshu Kumar died of suspected Acute Encephalitis Syndrome (AES), at Sakra Referral Hospital, Muzaffarpur Civil Surgeon Gayan Bhusan said.
The AES is a severe case of encephalitis characterised by inflammation of the brain. The mosquitoes transmit the infection causing high fever among the children.
All the three deaths have taken place in Muzaffarpur.
Earlier this week, a four-year-old boy died of suspected AES at the Sri Krishna Medical College and Hospital (SKMCH) in Muzaffarpur, about 70 km from here.
Bhusan said the situation is under control and there was no need to panic.
Last week, a one-year-old girl died of suspected AES at the hospital in Muzaffarpur. It was the first death suspected to be caused by AES reported in Bihar.
Bhusan said that so far 16 children with symptoms of AES were admitted to various hospitals in the district in the last 10 days.
"Every year, encephalitis hit Muzaffarpur, Gaya and other neighbouring districts in May or June. But this year it seems to have hit a little early," a health department official here said.
Last year, the AES killed nearly 240 children in Muzaffarpur and Gaya districts.
In April, the state government launched a special vaccination drive to cover 1.8 million children against encephalitis.