Joined second company unintentionally after joing first one. How to handle this situation?

I had 2 offers and I joined a big Company 10 days back. I had already submitted onboarding documents to the second company, and they created my employee ID the very next day after I joined the first Company. I told the second company to cancel my onboarding, but they say only termination can be done. In this scenario, what is advisable to be done? Will it affect my employment with the first Company if they get to know that I joined another company the very next day? I don't want to have dual employment because ethically, that's not the right thing to do.

I also have a third, better offer. Should I join this third company by sending an email to the other two companies, stating that I can't work with them?

Such vacillation between companies 1, 2, and 3 reflects very poorly on you. Decide once and for all which place to join. Every place will have its advantages and disadvantages; make a comparative study and decide based on your career goals. Some compromise may be needed, but then that is the position everywhere.

I have decided which company to work for. But what about the notice period? I cannot serve any notice period, nor do I want to pay for any notice period.