Morally, if you've signed up to a three month notice period, then you've little option but to accept the conditions you've agreed to.
If it's the job offer of a lifetime and you've been straight with your old employer and agreed to pay your bond and help out wherever possible, I can't see it's in your present employer's interests to hold on to you .... there is surely nothing worse than an alienated employee. In that position, if you felt suitably aggrieved, you could present any number of problems - e.g. short notice "sickies", finding tech. problems, being generally unhelpful etc.
If I was your employer, I'd let you go.