There's alot to whether or not those legs count on a grand champion certificate, too.
First of all, no less than 2 of the 3 legs you submit have to be won as either a senior or intermediate (6-8). Also, you can win one for best opposite sex or best opposite of variety, provided there are adequate numbers of rabbits and exhibitors showing rabbits of the same sex.
Another thing, and this one is liable to get some folks fired up--you have to have your rabbit registered in order for them to be eligible for grand championship. There are likely a number of reasons why, but I suspect the biggest one has to do with the ARBA being able to verify the age of the animal, and determine if the legs they won were awarded in the class they should have been shown in according to the date of birth on their registrations and pedigrees.
I've only scratched the surface. I just got my legs from the show I went to in Mississippi in mid-February. There are several different criteria which are listed. If you'd like, I will list them verbatim for you.