1) It depends upon which company you had worked and is it ISO certified company, does company has such type of reputation in the market.
2) Now its not like that all the companies follow NASSCOM rules. But if you have 3 month notice period in offer letter than you need to follow it. Thats it, there is no scope for legal action as they shifted notice period from one month to three month.
3) You can contact HR for your query, some company runs payment cycle 20 to next 19 and new joinee's salary are paid by cheque. You just have joined them two weeks before you get paid in next month.