Hang the Bandit
Having been in this situation before, ie. requiring to give 3 months notice, I would suggest you take advice from your union or solicitor. I was advised that despite what is written in your contract, legally, you are only required to give one week's notice. If your contract specifies a long time than this, then it is up to the employer to take action under 'breach of contract'. The worst outcome is the court could find in their favour and an order can be placed on you, stopping you taking up the new position until the end of your contracted notice period. The books I read on the subject say that this option is normally only used in rare cases, as the employer has to prove that they have lost out via your early departure. All in all, as others have said, it all depends if you want to run the risk of getting a reputation.