I also don't see that the OP is stringing anyone along. I've been doing the online dating thing for quite a while and have met some fantastic people. I do think, however, that it is more difficult to really get to know someone when you are only exchanging emails and it can take a while to make the decision whether this person is someone you do want to get to know in person.
I'm glad you just don't want to poof on someone. A polite email to #3 telling him that you do not feel you wish to pursue a relationship and thanking him for his time and wishing him luck in his search would be reasonable. For #1 you can tell him the same thing but suggest that, if he wishes, you would be open to continuing an email friendship and let him make the decision.