OS and RAID 5

I am in the happy position of having funds to buy a new DC/file server.
Is there any common wisdom about where to put the OS? Specifically, if I configure 3 drives with RAID5 (hardware), is it adequate to make a partition for the OS? Or should the OS get it’s own set of HDDs?
Also, if I only get 3 disks, and one fails, will the other 2 carry on until I hot swap a new one in or should I get 4 disks for no down time?