I accepted a job offer 3 days ago by an MNC. But then I just received a much better one today. What is the right thing to do?

Personally for me I would still go with the MNC because:

1. Integrity. I had already accepted - and it's not the best thing to go back on my word.
2. Like you mention, second company might be a dwindling business That said, of course the 20% higher pay is enticing, but with OT - what does that mean? If you are eventually going to work 30% extra time, would that 20% higher pay still be interesting to you?

I would also consider the job expansion prospects in both companies. See if you are able to grow in that role, and what are the opportunities they could offer.

If you haven't taken the first offer, would you have went ahead with this new offer? Considering the fact that you have both offers on hand at the same time.