The 3s is not active because of these 3 main comps, because of the ridiculous amount of PVE. Even retail was better during s11 because most pvp-ers didn't have the time to access the PVE gear.
Have you played during 4.0.6 here? - First week of the season 3s total arena teams were more than 300 with top at 2.2k, at season end 3s was reaching 3.1k. Queues at 2am, 3am, 6am. There were all kinds of comps Shattree, Godcomp, MLD, RMP, RLS, RMP, FMP, Fireplay, TreeSG, Thundercleave and many more comps that are extreme rare to see right now. I seriously miss these old times.