If you cannot use the Student/Trial versions, look into an license or a monthly subscription to the Product Design & Manufacturing Collection (which includes Plant 3D). This costs several hundred dollars per month, but is cheaper than a full annual commitment.
Is Autodesk Plant 3D free? Here is the reality. autodesk inc. plant 3d free
Autodesk does not offer a free standalone version of Plant 3D. It is a premium industry-specific tool for P&IDs, 3D piping, and isometrics. If you cannot use the Student/Trial versions, look