Scott,

When you put the 3rd card in you are introducing new hardware.

Check if all your cards can be detected and then write configuration to /etc/X11/xorg.conf.

aticonfig --lsa [this will show only 2 cards]
sudo aticonfig --adapter=all --initial [this will initialize all adapters]

If everything went fine, reboot the computer by “sudo reboot”.

After that, check if everything works:

sudo aticonfig --adapter=all --odgt

If you get something along the lines of "ERROR - X needs to be running to perform AMD Overdrive(TM) commands" because your SSH'd in, run these commands (which are in every tutorial out there)

export DISPLAY=:0
export GPU_MAX_ALLOC_PERCENT=100
export GPU_USE_SYNC_OBJECTS=1

miner@miner-001:~$ sudo aticonfig --adapter=all --odgt

Default Adapter - AMD Radeon R9 290 Series
    Sensor: Temperature - 41.00 C

Default Adapter - AMD Radeon R9 290 Series
    Sensor: Temperature - 41.00 C


Now, when you're this far, you should be able to run cgminer -n and get the 3 cards.

miner@miner-001:~/cgminer$ ./cgminer -n
 [2013-12-22 16:59:50] CL Platform 0 vendor: Advanced Micro Devices, Inc.
 [2013-12-22 16:59:50] CL Platform 0 name: AMD Accelerated Parallel Processing
 [2013-12-22 16:59:50] CL Platform 0 version: OpenCL 1.2 AMD-APP (1348.4)
 [2013-12-22 16:59:50] Platform 0 devices: 2
 [2013-12-22 16:59:50] 	0	Hawaii
 [2013-12-22 16:59:50] 	1	Hawaii
 [2013-12-22 16:59:50] GPU 0 AMD Radeon R9 290 Series hardware monitoring enabled
 [2013-12-22 16:59:50] GPU 1 AMD Radeon R9 290 Series hardware monitoring enabled
 [2013-12-22 16:59:50] 2 GPU devices max detected

You should see "hardware monitoring enabled" and you won't get those ADL errors.