Go ahead and add some. It probably didn't hurt at all to have it be dry for three days. The fermemtation will push CO2 out, preventing much oxygen from getting in and there aren't many bacteria I know of that can climb into an airlock! Things like fruit flies can be a nuisance though. When your pouring, just be careful not to spill any into the pipe. You can use cheap vodka too if you want for an extra measure of sanitation.