Till what time do I hide my new employer

So I switched my company recently, pay is decent. But I am not liking it here. Just had an interview with another company in 15 days of joining the current(switched) company and got to know that I couldn't clear the last round :( I told HR that I am still in my previous company, I have 2 other offers but still looking out as I have some days before joining the new company. Gave sugar coated answers as to why I want to join you guys Till what time can I tell HRs that I have not switched yet, and have other offers. Soon 1st month's pay will be credited at the new company and it'll start showing in PF in background checks I guess? If I tell the HRs that it's been a month at new company they'll ask shit load of questions why you want to switch so early and blah blah