To be fair there are a couple of gtlds and CCtlds that are making 1 letters and 2 letters available. For example if you want to get a 2 character .st you can at nic.st, this will cost you 999euros tho. If you want a 1,2,3 letter .ws you can at dyndot if you have $49500.00 to spare, at a quick look j and g were available. Rumour has it that .mobi and .coop are also going to release them.
Theres some being sold in the high price sub forum just now.
As for your question the third level single letter will be worth less than say a 2nd level. But they are still very memorable if your using them in the country.. out with the country of origin I personally think they are more of a domain collectors piece. But depends what country and how much you are paying for them.