You are going to have to do some revisionist history to teach this to third graders, I'm afraid. The true history of Valentine's Day is very violent and not suitable for this age group. See the link below for an explanation.
I would start with the history from the 17th century in Great Britain, since this is the closest to how the holiday is celebrated today. At that time, not only lovers, but friends, chose this day to commemorate their friendship (or relationships) by making small tokens of affection. Later, when printing became more available to the masses, this changed to cards. You will probably want to emphasize the FRIEND aspect, not the LOVER aspect for young children. You can tell them that sometimes, people didn't know what to say to tell a person that he/she was such a good friend, so it was easier to write it, or to give a gift, or candy, or flowers. That way, you could emphasize that actions can speak as loudly as words.
I would try to make it as light as possible. You can ask the kids to brainstorm ideas - what do you think some children might have made to give to their friends back before we had written cards? This would be a fun activity. You can bring in some props to stimulate their thinking -- like stuffed animals - a frog, perhaps. You can ask, "Do you think a boy might have gone out to the pond to catch a frog to give to his friend?" Or bring in some worms. "Do you think a boy might have given some worms to his friend to say, 'you are my good buddy'?" Or you can bring in a frilly hankie or scarf and say, "Do you think some girls might have made something like this to give to a friend?" If you think it is appropriate, you can get into the boy/girl thing because even at this age, kids are aware of the opposite sex. "What do you think an eight-year-old boy would have given to a girl to say he liked her before there were cards? Do you think he would have picked some flowers from his mom's garden?"
You can then have the kids make non-traditional Valentine cards or draw pictures of non-traditional things, not just hearts, roses, etc. This would be fun, I think. Or, you can assign them to bring something from home that they could give to a friend to let them know how much they appreciate their friendship. Make sure to set guidelines - nothing living, nothing gross, etc.