I'm presuming he's given a valid reason for being stored for 3 years?
ie, Was letting the heat cool of a bit after it was nicked!!,
or, Was used in a smash n grab, so couldnt ride it about for a while!!,
or equally as sinister, - Couldnt afford to be spotted after the drive by shooting me & me m8 did in town!!
In all seriousness, could very easily be nicked. check out very carefully, do all numbers match the log book? go round the bike with a pen & paper, jot down every number you can find, check them against the log book. Insist that it is HPI'd, check No. plate bolts. If dirty, prolly havnt been changed, if clean as a whistle, been changed/plates been off - why?
If you test ride - Be a bit carefull with the tyres too, stored for 3 yrs = could be knackered rubber....
If HPI is clear, & you're happy all is above board.... buy it, personally i would look around though, cos that doesnt sound mega cheap.....
for further info on birds, check out -
http://www.bikersoracle.com/blackbird/forum/index.php?