I have one months notice period stated in my contract.
If I handed my notice in with a 3 month notice, would they be able to refuse and hold me to a one month notice period?
DISCLAIMER: I'm not a lawyer, and the correct answer depends on both the exact terms of your contract, and the laws of the country you live in.
In general, I think that your employer could choose to terminate your contract immediately, taken into account the notice period of one month. Therefore, handing in your notice three months in advance is a risk.