I had a friend a long time ago who had a green burm that had gotten out of it's cage and ended up on the porch in the back of the house. Well, it was winter season and there was a cold front that came thru.....His wasn't as lucky to have lived thru it. It to was right in front of the sliding door as to be saying It's cold out here let me in!

It was too late though...

Incredible that you found it though after 3 weeks! IF it were mine I'd take it to the vet. I'm sure it's enjoying its nice warm dry cage though!