Why am I receiving an email from Blockchain about receiving a payment?
You aren't. It just looks that way, superficially.
received an email from Blockchain
It is easy to fake the "from" address in an email.
If your email app lets you view the full raw email headers you can probably see that this email followed an unusual path to reach you. But you don't need to do this. Businesses like Blockchain don't send emails to people who are not already their customers.
Authorize 3 network confirmations to add incoming 3.86655869 BTC to your Bitcoin balance.
What does this mean?
It is nonsense. It looks like a phishing email where someone is attempting to trick you into some sort of scam. Either getting you to divulge account details which will allow them to steal money or getting you roped into some kind of advance-fee fraud where they persuade you to pay them a release-fee or some other invented nonsense that is plausible to the inexperienced.
I would delete that email and forget about it.