OEM version of Windows has restrictions.
OEM license is good only for the machine it's installed on.
It has no transfer rights.
You do not get 2 free support instances from Microsoft as you do with full packed product [retail product].
see this MS articlehttp://www.microsoft.com/smallbusiness/buy/software/new-pc.mspx
and this one
http://www.microsoft.com/resources/sam/what_basics_licensingOptions.mspx
Don't confuse media [CD] with licenses. You can use the OEM media and buy a new license to use with that media and still validate the install [but the cost typically is the same so why bother]. However it's still OEM with the new license. To remove the OEM restriction you would have to upgrade. If you use upgrade software, you need the OEM disk. If you upgrade the software with FFP, you need the FFP disk.
MS has this thing for 5 licenses. 5 is the # where you achieve the first price break for volume licensing. Anything less than 5, Retail is the best overall value with Microsoft software. OEM is cheaper but know what OEM is before you buy.
So for your requirements of 3 computers, you could go retail and get 3 media[CDs] and 3 licenses all 3 having 2 free support instances with Microsoft.