There is now a 3v3 Full Share Mapgen-only matchmaker queue on the FAF client, and players have already started playing on it. Players have an additional rating for this queue, with the initial ratings for it based on 4v4 TMM ratings with additional sigma (more uncertainty, so the shown rating is initially lower and more easily changed).
Here is the initial map pool:
It uses the average rating of the players in a match to determine the bracket, and the brackets are currently not cumulative. The map sizes, slot counts, and number of mapgen settings sets in each bracket can be adjusted based on player feedback and how the queue goes. Suggestions are welcome. Feedback can be given here or in the Matchmaker Pool Feedback Thread.