I faked my asthma in 3rd grade and my parents still don't know.
So what happened when you faked your asthma in 3rd grade?
They didn't do or say anything until today.
It is wrong to lie to you parents no matter what. It is especially wrong when you lie and fake a medical condition.
I managed to give my folks the slip, don't worry. I don't think they'll blame me.
Well, I still have to reiterate that it's wrong to lie to anyone, especially your parents, about anything, especially a medical condition.  However, since what has happened is so far in the past, and there aren't really any repercussions today, it's probably okay to just let them forget about it.  But try not to be dishonest in the future.