S Kalmuthu was on his morning walk when the trio, who came on a two-wheeler, attacked him with iron rods and fled.
Kalimuthu sustained severe injuries on his head and legs and passersby admitted him to a private hospital where his condition was stated to be serious, police said.
Two special police teams have been formed to nab the culprits.
Protesting the attack, around 300 employees of the municipality staged a 'road roko' here for an hour.