The militants lobbed the grenade at the house of Y Jitendra, the pradhan of Kongpal Laishram Leikai area last night when Pradhan and his family were resting.
Y Ashalata (29), her daughter Tania (6 months) and her sister in law Y Robita were injured, the sources said.
They were taken to J N Hospital at nearby Porompat where the condition of the three persons was stated to be serious.
The area where the incident took place is 8 km from here.
The militants managed to escape.
No individual or group has claimed responsibility for the incident.
Jitendra's family members said they had not received any threat from any organisation.
Police said they would investigate whether it was related to any monetary demand or personal rivalry.
