That's not common. Usually it's the standard 1 month
It looks like it might be quite common
Seen some people say it's not enforceable, what does that mean exactly if you leave after say 1 month without agreement can they dock pay or anything like that?
Source Jobsite:
If you wish to leave before the end of your contractual notice, in practical terms, your employer does not have a great deal of options. Your employer cannot force you to work, even though you may be in breach of contract. In certain circumstances, your old employer may, however, be able to obtain an injunction to stop you working for your new employer during the notice period, but they would have to show that the new employer is a direct competitor and that there was a legitimate need to protect your old employer’s interests. Such claims are likely to be brought against senior executives only. - See more at: http://www.jobsite.co.uk/worklife/notice-periods-questions-answered-10523/#sthash.HLsFvYq9.dpuf
It could be looked at as a good thing that you are not quickly replaceable but 3 months is a long time for a new employer to hang around for.
Do many people get put on gardening leave or is this a thing of the past?