After installing AECOsim Building Designer Ss5 version 08.11.09.750 (non-Engish), I kept getting the error "Cannot load library" when attempting certain operations on my Windows XP system. I tried uninstalling Ss5, but then my original Ss3 version no longer recognized DWG files to be opened or referenced.
All versions of AECOsim Building Designer after 08.11.09.747 are no longer tested for Windows XP, primarily due to Microsoft ending support in 2014. This is likely why you had experienced the "Cannot load library" error.
All post-.747 versions of AECOsim Building Designer must be installed onto Windows 7 or higher.
Regarding the Ss3 installation no longer recognize DWG format, it's possible that by uninstalling Ss5 some part of RealDWG that Ss3 relied on was also removed. Reinstalling AECOsim Building Designer Ss3 in order to get the required components back onto the machine may help.