How to add minimum 3 serial ports to a Raspberry Pi?
If you want a ready built module from FTDI:

http://apple.clickandbuild.com/cnb/shop/ftdichip?productID=116&amp;op=catalogue-product_info-null&amp;prodCategoryID=89

Otherwise build around the FT4232H.
(Moved this to an answer so you can mark it accepted)