python3.dll
This file is not a system component: the python installation, not a download. Restore it from the source that installed it rather than downloading a replacement.
If the file is already there and Windows still complains, read the message again: “entry point not found” is a different problem from a missing file, and installing the runtime again will not solve it.
And if the box says RUNDLL rather than naming a program, the fix runs the other way: something left an instruction behind pointing at a file that is meant to be gone, so restoring the file is the wrong move.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
The same goes for the tools that offer to do it for you: what a DLL repair tool can and cannot actually fix.
What it is
The library that carries the Python interpreter on Windows. There are two shapes of it: the version-specific one, python313.dll or python39.dll and so on, which holds the interpreter for that exact release, and python3.dll, which exposes only the stable subset of the C API so that a compiled extension can work across several Python versions without being rebuilt.
Where you see it: Programs that embed Python, compiled extensions and packaged tools built with PyInstaller or a similar bundler, usually right after a Python version was changed, removed or installed alongside another.
Version and architecture both have to match exactly, and that is the whole problem. Something built against python313.dll will not load python312.dll, and a 64-bit build will not load a 32-bit interpreter no matter how many copies of the file you place next to it. So the fix is to install or repair the Python release the program was built for, from python.org or the Microsoft Store, rather than to obtain the file on its own. It is worth knowing that python3.dll is deliberately limited: it forwards only the stable-ABI symbols, so an extension that needs the full API will still fail against it even when the file is present, and the answer there is again the version-specific interpreter. If you have several Python versions installed, check which one the program is actually finding before changing anything, because a PATH left over from an old install is the most common reason a machine with a perfectly good Python reports this error.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.