Resignation and notice period length under probation

Hii, i was working with a firm a month ago and during probation I resigned . Post my resignation i requested company to release me asap or in one month notice period so tht i can join another firm in the stipulated time period. However HR and Hod of my dept did not agree to this and forced me to finish 3 months notice during probation(coz they cud not find a replacement in one month) which i was not ready to serve coz my employment letter clearly stated that post probation only I shall be liable to serve 3 months notice. Also there is no term defined as "resignation under probation" in my employment letter so basis that I got ready to serve only one month notice. On the last day of one month notice period, i gave company assests at reception (coz HR was not ready to complete the exit formalities) and dropped an email abt it to HR immidiately. Now post my departure from the company, it has sent a legal notice to me (not on stamp paper) stating that I shall go to do the handover again else it will be considered as breach of contract which I feel is very inappropriate. Kindly advise