Voting begins in Tanahun- 1, Chitwan-2 and Bara-2.
In Tanahun, voting has started in all 152 polling centres under the constituency, said Hari Prasad Dhakal, chief of the district election office, Tanahun.
The constituency has a total of 123,305 eligible voters.
Nepali Congress candidate Gobinda Bhattarai is scheduled to cast his vote from the polling centre at Tanahun Basic Secondary School in Byas Municipality-11.
CPN (UML) candidate Sarbendra Khanal and Rastriya Swatantra Party's Dr Swarnim Wagle are deprived of exercising their franchise as their name is not listed on the voter roll.
Nepali Congress's Ramchandra Paudel vacated the House of Representatives seat after being elected third President of the country.
In Chitwan, voting has begun at all 138 polling centres of 43 polling stations, said assistant returning officer Ishwori Acharya.
The constituency has the total 124,695 eligible voters (63,291 women, 61,403 men and one third gender).
The main competitors from the constituency are Nepali Congress Jit Narayan Shrestha from the democratic-left alliance, CPN (UML)'s Ram Prasad Neupane and Rastriya Swatantra Party's Rabi Lamichhane.
Of them, Shrestha is casting his vote from the polling centre at Chartumukhi Secondary School at Shaktikhor while Neupane and Lamichhane are missing in the voter roll, it has been said.
Lamichhane vacated the House of Representatives in a case of citizenship.
In Bara-2, around five percent of voters have cast their vote in a period of one hour, according to chief election officer Mukesh Upadhyay.
A total of 138 polling centres for 82 polling stations have been designated.
Of them, 65 have been categorised as highly sensitive, and 17 general, according to the district administration office. The constituency has the total 105,248 eligible voters (58,934 men and 46,314 women).
A total of 34 candidates are running from the constituency for the House of Representative seat.
Voters can cast their vote until 5 p.m. today.
According to the authorities, tight security arrangements have been made for the by-elections.