I am currently researching this for my 3Character.com site and don't have a complete answer for you yet.
The market for 3 mixed character domains is recently new, (as is the LLL and NNN markets), so I haven't been able to get sales figures for much other than LLL/NNN .com, .net and .org.
GreatDomains just started listing these over the past couple of months which is one indicator that they have a good percieved value and alot of potential. An example of what the prices are at GD for 3Character/mixed:
4hd.com $15,000
ok4.net $8,500
69m.org $8,000
4md.net $7,500
4rx.net $7,500
As I retrieve some more info, I will update here. If you are seriously interested, PM me and I will update you and fill you in w/more details over the coming month. I am still struggling w/ php, which has put me behind schedule in getting the 3Character site up.
PS Many of the good .com mixed3char have not been available for some time. IMO, the market is just starting to get wings, due to the popularity of 3Char names. They are short, easy to remember, easy to type, and if they carry a meaning, they are strong candidates for branding.