I got a Blockchain wallet and and someone sent me 3.5BTC (or tried) and he tells me that I need to atleast have 0.12 BTC
What does mean you
got? Does it means someone give you blockchain dot com wallet access and balance showing there? If so, then likely someone want to cheat you. Most likely scammer import some btc address "
only watch" mode. And it won't spend without private keys. Even you deposit on that wallet scammer will spent it before you. There is not something like that you need to deposit for withdrawal. It's not ponzi scheme, this a web wallet. Even I will suggest don't use web wallet, follow first reply from above and install electrum and if you want to receive fund use electrum. It's more easy than blockchain but you have full control.
If really someone sent you found and you believe it were failed then check your address on
Block Explorer.
Just a reminder: Don't send any funds to a unknown person if you are not fully aware where you sending or don't make any deposit if you don't know the site very well.
Edit;
Just reported to Blockchain support. Its a scam. I wonder why no one reports the Bitcoin Makers from youtube from scamming people.
Can you please share more details? For your info, don't trust all fucking YouTubers. Most of them are scammer. Don't find quick rich method, sure you will be scammed.