Amapi 3D format (.a3d) is somewhat old and the program itself is discontinued for years so you will not find many current programs that still support this particular file type. However, some dedicated converters like 3D Object Converter should still support it and allow you to convert it to something else, for example to the STEP product data format (.stp, .step).
Open A3D file Open STP file3D viewer and converter with huge file type support