Hi Pnm,
Was the undertaking on a stamp paper? I guess not, looking at the nature of the statement. So, don't worry, they can't do much besides sending you such letters. Moreover, as I mentioned earlier, your bond/undertaking tries to secure the employment of the new employee for a minimum period of 3 years. In law, this agreement is void under Section 27 of the Indian Contract Act. You cannot restrain the employee from leaving the job unless the employee was a trainee and the company had spent a large amount of money on his training.
Since there were no training costs involved, nor is there a mention of an exit clause in terms of payment of costs, this is just a one-sided agreement and an infringement on the right to employment. To be on the safer side, speak to company personnel and hint that you would consult a lawyer on harassment. It works. Sorry, I couldn't respond during the day as I don't get the time to check the site.
Regards, Rajat Joshi