I have a carbon 16 in console and 12 up front. Want to add a 12 next to console for mapping. Also what box and cable do I need to tie all 3 together? Thanks

Where to buy is a personal choice. Some go for cheapest price while others prefer to pay a bit more but get better service after the sale. As far as connecting the units together, if the 16 and 12 are currently networked then you should just need to add an additional Ethernet cable. If you currently have any NMEA devices, then you will also need an additional NMEA drop cable and T-connector.