3) Cayman S
All around, the M3 is a better car than the Cayman S, and on the track they're neck and neck. "All around" as in, space, comfort, amenities, etc. R8 I know nothing about, except it looks awesome, is overpriced, and the performance numbers are pretty bad for the cost, but if used they're relatively equal in price then go for it. What about a GT-R? Those are cheaper than an R8 brand new, and the performance numbers are amazing.
2008 M3 E90 6MT - Jerez Black - Premium Pkg - Technology Pkg - Heated Seats - Enhanced Premium Sound