I thought this achievement was bugged for me, as it has been for others. I found out that the Weapon Form Level 3 Astral Constructs do not unlock until you get a specific material. It is the Untapped Potential asset. All level 3 weapon form blueprints require one Untapped Potential asset.
I got my first one by completing the Fridge Duty side quest. After defeating the Former boss, I got one Untapped Potential asset and all the level 3 constructs finally showed up.
I have not done any of the other side quest boss battles yet, but I am going to guess that this is where the rest of the Untapped Potential assets are gotten to upgrade your weapons to level 3.
I also got one Untapped Potential asset during the main story quest 9, Polaris.