If I remember right the 3 is for motorcycles and the 7 is for snowmobiles they have different internal parts as listed above and I am no 100% sure if the bodies are the same, that's a question for someone who knows more than me.

If you tear it down and find out what parts are in it this should tell which one it is.

You should be able to find information about different set ups here:
http://www.sudco.com/Carburetor01.html