I'm sorry hun, she was a gorgeous girl. I'm glad you are doing the right thing and letting her go peacefully before her health declined too much. Megacolon is a very painful condition, and no 3 week old baby should have to suffer through it.

And this is also a good lesson for people who like to breed high-white rats(blazed, husky, masked, any headspots, etc). Megacolon happens and is a serious concern. If you are breeding high white rats, and get megacolon babies, it is very important to not use any rats from that line for breeding, as they carry the predisposition to pass megacolon on.