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.
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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.