There is no official .NET Framework 4.3. Download the nearest real version (4.5.2, 4.6.2, 4.7.2, or 4.8) from Microsoft’s official site. If you share where you saw “4.3” required, I can help you trace the real dependency.
He dug through his ruggedized laptop bag, past the spare thermal paste and the half-eaten granola bars, until his fingers brushed against a worn, silver thumb drive labeled net framework 4.3 offline installer
https://dotnet.microsoft.com/en-us/download/dotnet-framework/thank-you/net48-offline-installer There is no official