I search a way to convert a Group 3 compressed TIFF to png or for best in pdf with c#.net.
Try LibTiff.Net library for this.
The library comes with tiff2pdf utility (you would need to build it yourselves) that probably does exactly what you need. You may even incorporate the code of the utility in your application.
The library and utility are free and open-source. License (New BSD License) allows any modifications. You can use the library and utility in commercial applications.
Disclaimer: I am one of the maintainers of the library.