After 4 days of joining I received offer letter from dream company - what to do?

I was expecting an offer from my dream company, but they were delaying it, so I joined another company. Now, after 4 days of joining, I received the offer letter asking me to join in another couple of days. I am happy, but now I have joined another firm with a notice period of 3 months.

What should I do?

What do you want to do? Does your dream company know that you have joined a new company? Think over..

No, they don't know. I was thinking of joining another company and leaving my current company. Anyhow, I do not need an experience letter from my current company as I have only worked for 4 days.

Can they take any action against me? I cannot serve the notice period, and they are not ready to release me early.