Accidentally Joined Two Companies: How Should I Handle This Ethical Dilemma?

I had two offers and joined a big company 10 days ago. 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 said 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 find out 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?

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.