A girl I've never met before sent me a message through Instagram asking if Id like to make 20 grand. Naturally I said "who wouldn't?". She then laid it out for me. She said she manages access accounts at Fidelity investment. And that all I have to do is give her my mobile banking login info and she will transfer 3 grand into my account from a non-profit investment I then transfer back her cut and the next day another 4 grand. She said I don't need any money on the account. Is this a scam?!
Yes, of course it is a scam.
Take a step back and ask yourself what on earth would make someone who really worked at an investment company (a highly regulated industry in itself with tight scrutiny and controls on what is being done) would contact someone they don't know with an offer of free money.
Even if you worked in an investment company and found a loophole in their processes, would you contact a complete stranger to help you out with it?
You did the right thing in asking here but next time just ignore and block.
Yes it is a scam. This anonymous person will make a fraudulent transfer to your bank account. Your balance will be $3,000 higher until the bank registers it as a fraudulent transaction when your account balance will drop back down by $3,000. Meanwhile you make a non-fraudulent transfer out of your account to this person. In the end, you're scammed out of whatever money you transfer out of your account.
Check out some of the similar questions on this site and you will see the same thing happening.