Microsoft provides "Full Package" installers that include all components, though these are typically intended for older OS versions like Windows XP or Vista. Microsoft .NET Framework 3.5 Service Pack 1 (Full Package)
You downloaded the x86 version but are running on x64 (or vice versa). Fix: Download the combined x86/x64 full package. It is a single executable that detects architecture. net framework 2.0 and 3.0 offline installer