I've updated the Homicide: Life on the Street Video FAQ with details on the Season 3 DVD set that will be released on October 28th. Including a picture of the cover art and a list of extras. The set isn't yet available for pre-order, I'll post when it is (of course).
You can still order Seasons 1 & 2 on DVD, the Homicide reunion movie was also marked down not too long ago. If you haven't seen the series or much of it, I heartily recommend you track down the season 1 & 2 DVD set (buy it, borrow it, rent it) and give it a shot. You won't be disappointed.