I thought I would post this since there seems to be a lot of unsure/bad info out there as far as programing a third key for a 2011 Wrangler. I even got all kinds of different info from my 2 local Jeep dealer service departments.
I am not a dealer or a mechanic or anything of the sort, but I did all this myself today with great results.
So here is what I actually found out (and did).
I have a 2011 Wrangler JK unlimited sport without power locks. So this info is for a basic key with a chip. (no info on keyless remote key)
Yes you can buy blanks on-line. I paid $20 including shipping for a gray generic key with a transponder.
The website said it was good for Wrangler's up to 2010. The key does work for a 2011 - at least it worked on mine.
You can get it cut at Home Depot or Lowes - you do not need to have the dealer cut the key.
You can program it yourself at home as long as you have two working keys still.
put in your first key and turn it to "on" position (don't turn on Jeep) and leave in "on" position for about 10-15 seconds.
turn first key off and do first step again but with your second key and leave in "on" position for about 10-15 seconds
turn off 2nd key and put in your new key and turn to "on" position like the last two, leave in for about 20 seconds.
remove third key.
third key should now be programmed.
Hope this helps.
Worked for me.