COMSOL does not host public downloads for legacy versions like 3.5a. To get a legitimate installer, you typically need an active account linked to a valid license.
If you require COMSOL Multiphysics for professional or academic work, the recommended approach is to avoid the "free download" rabbit hole and utilize official channels: Comsol Multiphysics 3.5a Free Download
with installations as old as version 3.5a, meaning your current license may still be able to run the legacy software if you can obtain the installer. Free Alternatives and Trials COMSOL does not host public downloads for legacy
Version 3.5a introduced several enhancements over its predecessors: Comsol Multiphysics 3.5a Free Download