Each wallet is stored in its own file on your PC. You can locate your wallet files here: http://docs.electrum.org/en/latest/faq.html#where-is-my-wallet-file-located. Actually, the FAQ is confusing when it comes to Windows; to find you wallet there, open File Explorer, and copy and paste this into the address bar: %appdata%\Electrum\wallets

With Electrum closed (very important), rename the current "default_wallet" file to something else, and then rename your imported old wallet to "default_wallet".

Maybe we can help if you can give us a detail of what happened, OP.

Also, when you open electrum it will actually open the last wallet you have open when you close electrum.

OP, let us try to help you get your 3 BTC back if possible.  We are here to help if we can.  You don't even need a default wallet with Electrum.  As noted in a post above, the last wallet you have open in Electrum will open automatically when you mount Electrum the next time.  Lets say you have a wallet named XYZ open and shut down Electrum.  In essence that becomes the default because it will mount next time when you mount Electrum.  Now, if you are CERTAIN the default wallet is empty/useless/no coins/etc.... then you can go into the folder containing your wallets and delete the "default wallet".  Now it is out of the way and you will never accidentally use it again.