Quartex: Hard to say, they did a bunch of GBA releases in '01/'02 and a couple of Apps last year, definitely not active but not confirmed dead either.
Skid Row: Last seen releasing trainers for PC, site at http://skidrow.dk/
looks fairly well abandoned.
Tristar: Merged with Red Sector Inc. in 1990, some releases on GBA, PC + an aborted attempt at a DVDR section last year. Most likely sleeping cos 'Legends never die'
A little TRSI related trivia, Muffler of Scoopex put out his first CD on TRSI/Fairlight Records, he is now very well known in the Drum & Bass community having released dozens of records for a variety of different labels.