Adding 3TB drive on RN104 - Taking more than 30 hours

I guess your original volume is created with one drive,then you added second one...third one.
please note if the volume has changed from raid 1 to raid 5,all data will be re-striped,it is not the same as simply resync(make a mirror disk in raid 1),restriping will take much longer time to finish,so you don't need to worry about the speed.

Despite what de niro says it sounds too long to me if it eventually takes 60 hours, having said that I would wait for completion and see if the process completes satisfactorily.
If it does not then I would contact support.
What firmware are you running?
What disks are you using?
Are you using xraid?
Make sure you have a backup of the data.
Check the smart attributes of the 3rd drive in the GUI. Even if it does rebuild, I would suggest powering down after rebuild is complete, remove third disk and test it using vendor tools on your PC. Leave the nas off while you do this.
If the disk is OK, reinsert it into the same slot while the nas is off, then power on again.

I just did something similar with my RN104, and I'm worried something went wrong. I had a 2x2TB raid 1 set up, and I just added 2 more 2 TB drives. Now I'm unable to access my admin page, and RAIDar has a green light, but isnt reporting any status (volume, disks, fans, UPS)

Is this normal?

Do you see anything on the LCD on the front of the 104? If not, if you momentarily press the power button does the LCD turn on and display useful information?

Do you have SSH enabled?

SSH is not enabled, and the LCD screen says the same thing its said for coming up on 18 hours now - "shutting down - goodbye". Shutdown was the last command I sent it yesterday once everything else reported the device offline. At the time I took it for a good sign that it was rebuilding and would shut down once finished.

You are saying you told the NAS to shutdown while it was resyncing?
The device should have been online while the resync was taking place.
I think you should contact support, or if you have a backup you could force a shutdown by pushing and holding the power button. Then see what happens when you reboot. If a resync was interrupted, it should continue.
The missing info in raidar is a known problem so don't worry about it.

Well that's the thing, I have no idea if it actually is resyncing or if its just hung- trying to access the admin page or even browse files gives me "the device is offline" errors. Ignoring those errors shows me the device, but the only info that populates is its reported time, and the only options available are Shutdown and Restart. So I went with troubleshooting 101, "Have you tried turning it off and on again". If a resync will continue after a shutdown I might pull the power if it hasn't resolved itself by tomorrow. If the raidar info is a known issue I figure I'll give it at least that long. Any idea how long a 2x2tb Raid 1 should take to rebuild to a 4x2tb raid 5?

If the display says shutting down then the unit has hung and is not resyncing. The device should function ( albeit a little slower) during a resync and the GUI should be accessible. Your NAS is not in this state.
The 104 needs to be unplugged to force a shutdown.
The 4th disk may be the problem here.

Do you have a backup - this is also advised prior to any volume expansion manoeuvres.

Well, you were right, that did the trick. After reboot, the resync seems to be moving as normal. Disk 3 is showing as black though, and doesn't seem to be part of the raid. Can the system only support adding one drive at a time?

Yes, generally advised to add one at a time and let resync complete, however you can add more than one and they'll be processed 1 at a time.