Thanks everyone, I managed to get my 3 BTC back 
For future reference, all I did was install MultiBit on a seperate PC, copy/paste the backed-up multibit.wallet file over the new one created by the new MultiBit installation. I then hit the "Reset the Blockchain" button and within minutes the wallet was up to date and I had access to my 3 BTC again
For future reference, all I did was install MultiBit on a seperate PC, copy/paste the backed-up multibit.wallet file over the new one created by the new MultiBit installation. I then hit the "Reset the Blockchain" button and within minutes the wallet was up to date and I had access to my 3 BTC again