I've gone on three dates with a great girl who's smart, interesting, kind and pretty, but I don't necessarily feel the romantic chemistry. Should I call her and tell her this? Should I email her? I really want to do the right thing.
Not Feeling It
Dear Not Feeling It,
Cliched though it is, this actually may be the perfect opportunity to, in good conscience, use the line, "I think we should just be friends." You can say it's been great getting to know her, you hope to see her around, you wish her all the best, etc., and then leave it at that. You don't need to go into the part about not feeling the chemistry. She'll get it, and you avoid making her self-conscious by insinuating that she may have done something wrong or something to turn you off.
Look back at how the two of you have communicated so far to see whether it makes sense to communicate this by email or phone. If you've only ever talked on the phone, an email would be impersonal and inappropriate. If, in fact, you've made all your plans via texting, you have more free range to use any mode of communication. (If you'd been on more than, say, four or five dates, you'd have to call or even have this conversation in person, but you have more leeway based just on the short duration.)
Before you move on, though, I have to point out one thing in your question that caught my attention. You say you don't "necessarily" feel the chemistry rather than that you definitely don't, so it might be worth another date before you close the door entirely. Things can change and develop, but only if you give yourself that opportunity. Since it sounds like you are compatible in a lot of ways, it may be worth giving her another chance, especially if you get the sense that she's interested and/or that she may holding back on opening up to you completely.
If and when the time comes (and you're right, it may already be here) to end things, treat her as the smart, interesting and kind person you've found her to be. Don't overexplain or apologize, and prove yourself to be smart, interesting and kind as well. That will be the right thing.