Whatever you do, when you do fall in love with her and want to say it back, don't do it in the middle of sex.
Just be honest. I've been the first person to say those three little words in two of my relationships; in the first I said it knowing full well he wasn't ready to say it back, but I wanted him to know. He said that he didn't know when he could say those words back to me and that he cared for me a lot. He said it two weeks later while we were having sex (did I mention it was my first time?). By contrast, when I told my SO that I thought I was falling in love with him, despite the fact we'd only known each other a week, he said he felt exactly the same way.
I didn't know what he would say when I said that to him. I just hoped that whatever the response, he would be up front and truthful.
Ultimately, that's the key. You ought to be gentle, caring, and honest in your response. Be considerate for her feelings. She might know that you're not ready to say it yet. You never know.