Every partition has its own (hidden) boot record which would explain 2 (C:, and D:), but that total would also include any Virtual Machine which could explain #3. Do you have one installed? Or, if this is an OEM system, there may be a hidden recovery partition and perhaps a McAfee software update has made it detect boot records in a slightly different manner, but that's just a guess.
I did find a McAfee user posting a similar query, but with Windows 7, here: http://www.sevenforums.com/general-discussion/267524-3-boot-records-detected-ok. html
I don't think it's anything to worry about, but a Vista forum might be of more help, ther's a good one here: http://www.vistax64.com/ or Technical Support might be able to explain it if you are really deperate to go through that experience ;-)