Depending on which contraption you initially choose on Eva's Hammer, the other two will be hidden somewhere in the game that you can go back and find.
YouTuber "randomChievos" gives and excellent visual guide about where you can find all three contraptions and their upgrades.
Each contraption requires you to do a small side mission revolving around the device, all of these can be started on Eva's Hammer. The "Battle Walker" mission is started by talking to Irina in the Kreisau Club. For the "Constrictor Harness" you need to talk with Habib by the Cantina. For the "Ram Shackles" you need to talk to Ritva just outside the Helm. Keep in mind you need the contraption before starting the quests.