Spit is inefficient way of transmitting dieses. Think of it this way the spit you have is pretty neat. For example if you think you are going to throw up or feel like it your body starts to salivate more heavely this is to protect the teeth
from the acid in your stomach. Also spit helps break down food and other items. So its not this great and wealthy area for all STD's to be in.
I won't speak for other STD's because really this isn't the place for all the other STD's. But HSV is a skin-to-skin virus. It doesn't live long outside of the body. Low risk doesn't mean no risk just means it is low. I don't think I have heard or the experts have heard of someone getting an STD from spit. At this point if you haven't had any symptoms in 3 weeks I would say its time to think otherwise. You have not much to worry about here. Get some sane thoughts. Think of eating ice cream and cake. It always helps me.