Is it resigning from tcs or any company is a good option before even not having any offer letter into my hand

Better to have offer in hand and then resign.

You can if you are very good in your skills and job is available for you. In case you do not get job within 90 days and there is long gap between job switch, you may have to carry the explanation in your subsequent job interviews.

Yes you can, but just ensure you have decent savings. If you have nothing in the bank, I wouldn’t recommend taking this step. But if you have decent savings to manage your day to day living expenses for atleast 6 months, say, then hell yeah I would say go for it 👍👍👍