How to get relieving letter from co. after serving part notice period?

It all depends on your Employment Contract. Most of the Contracts allow for buying out the notice period, just check if the same is present in your contract or not.

In case such a clause is present, you should send them a written letter of  resignation informing them that you shall be serving a notice period of 1 month and shall be paying them for the other 2 months and that you be relieved within a month from the day of your resignation letter.

Based on their response, you can pursue the appropriate remedy. 

You can file a case before the Labor court if you come within the definition of workmen or in the competent civil court claiming your relieving letter as well as the full and final settlement.

Applicability of the Industrial Disputes Act (for workmen):

 
The Industrial Disputes Act extends to whole of India and applies to every industrial establishment carrying on any business, trade, manufacture or distribution of goods and services irrespective of the number of workmen employed therein. Every person employed in an establishment for hire or reward including contract labour, apprentices and part-time employees to do any manual, clerical, skilled, unskilled, technical, operational or supervisory work, is covered by the Act. This Act though does not apply to persons mainly in managerial or administrative capacity,persons engaged in a supervisory capacity and drawing > Rs.10,000 per month or executing managerial functions and persons subject to Army Act, Air Force and Navy Act or those in police service or officer or employee of a prison.  

Alternatively, you can file a claim petition before the authority under Payments of Wages Act. A labour court judge is generally designated for this purpose. Or alternatively, you can file an application under Section 33 C(2) of the Industrial Disputes Act.