3 I believe only seems to drop from the free chests and as every player under the sun is opening as many of these as they can with their characters, at the moment they're not worth very much (Diamond Aegis got hit hard by these chests too price wise). I'm pretty sure alot of people have listed these in sell and haven't had any bids on them due to over estimating their rarity and value.
But to be honest I'm not sure on value at all, especially for a req 12, seems to be in the hard to sell category like Igneous Blades etc. I'd say your best chance of selling it would be to put an auction up starting at around 5k and just be happy what you get for it. But get more opinions.
http://www.guildwarsguru.com/forum/s...rch=adamantine
Look at that link also (search results for 'adamantine') the amount of threads that have 0 replies is quite amazing, and the ones that have 1 or 2 replies seem to be bumps from quick inspection). Check out some of the threads that did get bids (preferably req 12 ones obviously) and base your price from those
