Hi,

i want to ask what is the best way to export from avid mc3 to After Effect CS4 and back? Important that quality won't lose.

It depends on what is the format of your original material and how you're going to output the final project. Usually it is safest and fastest to use QuickTime reference and the original codec to export from Avid. This will save time and the quality doesn't degrade. If the material is compressed (eg. HDV or DV25), AE will need to decompress it anyway before rendering anything. When you export from AE to Media Composer, it's best to think about the final mastering format of the project. If you're mastering on DVCAM, for example, you can use the DV25 codec. If your final output is digibeta, you will gain a little bit in quality if you export it as uncompressed instead. This will minimize unecessary decompression/compression cycles.

I think Your question is answer in this video.