I guess it would depend on what you mean by "3 disks". If I only had access to three physical disks, I'd set it all up as a single RAID-5 array and put everything on there. Performance will suffer, but it'll protect the data better than any sort of non-RAID setup. I'd rather have a slow but safe server than a slightly faster one that's liable to lose all my databases if a disk fails.

If you mean I have three allocated RAID-protected arrays on a SAN, and can't have that reallocated or whatever, then (b) sounds okay. I'd have to test and monitor for I/O bottlenecks, but it'd probably work okay. Tempdb will be slightly slower than I'd like, but on many systems with a properly configured SAN, you'll never see that in terms of actual performance metrics in the application.