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 two second company and they created my emlpoyee ID the very next day I joined first Company. I told second company to cancel my on boarding but they say only termination can be done. In this scenario what is advisable to be done? Will affect my employement with first Company if first company get to know that very next day I joined another company? I dont want to have dual employement because ethically thats not the right thing to do.

I also have third better offer. Should I join this third company by sending a mail to earlier 2 companies stating I cant work with them?

Such vacillation between companies 1,2 & 3 reflect very poorly on you. Decide once 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 notice period. I can not serve any notice period nor I want to pay for any notice period.