I think a decent counter for a triple Art Junker in a 3v3 is a double Merc Pyra and have the ship be parked just outside of the Artemis's range. The Junker will have a tough time trying to get in range as ships cannot move sideways.
For brawlers, they'll have to rely on tactics and terrain. One example is have a ship be bait, have it go to right forward side of the map where it can easily hide behind terrain if endangered, draw both long range ships' attention as much as possible. The other ship will have to go to the left side of the map, which will be ideally at both enemy ships' rear. Additionally, this other ship can also hug the ground to avoid the chances of being seen and rise up quickly to overwhelm one of the ship. The first ship can join battle when the ambush ship is engaging. Of course, this requires good co-ordination and is a little situational.