The Kangema, Kajiado North and Ndhiwa constituencies by elections will be held on September 17.
The Independent Electoral Boundaries Commission (IEBC) said Tuesday that polls for 12 civic seats will also be held on the same day.
IEBC announced that political parties wishing to field candidates in the mini polls should have nominated their flagbearers by August 15.
The parties will be required to forward the names of candidates to the IEBC by August 23.
The declaration comes days after National Assembly Speaker Kenneth Marende declared the seats vacant.
On Friday last week, Clerk of the National Assembly Patrick Gichohi confirmed that the writs were issued for the three parliamentary seats, which fell vacant following the deaths of Environment minister John Michuki, Internal Security minister George Saitoti and his assistant Orwa Ojode.
Mr Michuki died in February while Prof Saitoti and Mr Ojode perished in a tragic helicopter crash last month.
There was a holdup in declaring the Kangema seat vacant owing to delay by Parliament in passing changes that had been proposed to the Elections Act under the Statute Law (Miscellaneous Amendment) Bill, 2012.
The Act had taken away powers from the Speaker to declare the seats vacant and issue writs to facilitate the conduct of by-elections.
It did not make provisions for conducting by-elections, necessitating the amendments signed by President Kibaki into law early this month.