Sadly, the 5D3 focus screen is not considered user interchangeable. I tried to clean mine and messed up the crap out of it. It's a very soft, laser etched plastic and damages before you know what has happened.
That being said, you can find a new one for $15 and it really is not hard to replace if you are careful. I did mine and kept pulling it back out to blow off specks of whatever happened to float on it. At some point it was pretty much clean and I quit trying. Dust will get on them and just live with it if a blower won't take it off. It will not affect the image or camera operation at all.
Could you please advise. Where can you get for 15? I been seeing it for 40 bucks.