The accused, identified as Aftab Alam, who had divorced his wife in April last year, used to come to his in-laws' place at Mango here to meet his son but was never allowed, DSP K N Mishra said.
When Alam reached there this morning too and sought to meet his one-year-old son, his father-in-law Nurullah Sharif, brother-in law and sister-in-law denied him permission, he said.
An enraged Alam then brandished a knife and stabbed all the three, seriously injuring them, Mishra said.
The accused was carrying the knife probably to teach his in-laws a lesson if he was denied permission to meet his son this time too, the DSP said.
Alam was arrested, the police officer added.