Hey, 

First of all congratulations on your result. Each IIT releases its own GATE cut-off and it will vary for each M.tech programme and category of Candidates.

The COAP ( Common Offer Acceptance Portal ) is the registration platform for GATE, 2019 qualified candidates for IIT M.tech admission. Candidates have to apply individually to the college and also through COAP for IIT M.tech admission 2019.

GATE, 2019 COAP process will be held in four rounds tentatively.

Now, coming to your question, any of your three results can be used. It's totally at your discretion. As the result declared, it can be used for 3 years from the year of appearing in the exam. So you can use your every marke for fetching the seats in the good Colleges. According to me, it won't have any impact you opt to mention only one score and that would be the best of the three. Different IITs have  IITs different criteria of shortlisting their Candidates from the application form, so you shouldn't worry about it.

However, my sincere advice would be to opt best one of the three and also check out the stats as which marks would help you to fetch your desired course.

In case you are looking out for more further details related to GATE queries, please visit the pathfinder mentioned below;

https://engineering.careers360.com/articles/iit-mtech-admissions

I hope the above information would be useful to you. 

Revert back in case you have more doubts.

Good luck !!!

Thank you.!