Another 52 people were reportedly wounded in the attacks that struck at around 4:00 pm (1500 GMT), one targeting the fish market at Baga Sola and the other two occurring at a refugee camp on the outskirts of the city.
Lake Chad is shared between Nigeria, Niger, Cameroon and Chad.
Climate change has reduced its surface in recent years, but it contains many islands and islets used by fishermen and its banks have dense vegetation, which makes infiltrations by Boko Haram Islamists into Chad much easier.
The Chadian army has since the start of this year been involved in a regional offensive against Boko Haram, whose attacks have spread across Nigeria’s borders to neighbouring countries.
At least 17,000 people have been killed, mainly in Nigeria, and more than 2.5 million made homeless since the Boko Haram insurgency began in 2009.