As of March 1st 2013, 3Character.com reports:: 3-Number .net - $1200 (Unchanged)
A few 3 number sales for comparison would be:
711.Net - $3,977 - Afternic - 10/14/2012
711.Net - $1,368 - Sedo - 3/4/2013
^ Indication of a possible value decrease since last year
857.net - $796 - Godaddy - 4/23/2012
488.net - $500 - Sedo - 6/16/2011
979.net - $510 - Sedo - 6/15/2011
251.net - $1,000 - Sedo - 6/8/2011
Granted, it depends on the numbers, if the NNN match a popular 3 digit area code the value may go up substantially. Likewise if it's a popular reference number like 666 or 777, 420, 747, etc..
Just every day numbers with no special significance may be safe to look between the $500 to $2k ball park. That's just speculation though, I have no clue what you're number's are and haven't done much research to formulate a more accurate answer. So this is just a loose opinion
Eric Lyon