My manager is the same way. But we have an unspoken agreement that if I ever need her for anything, no matter how small it might be, she's always available by her cell phone. There are very few people who have this number, and I'm the type where I don't call her unless it's absolutely necessary.
That aside, has there been legitimate reasons she hasn't been there, such as a family emergency or something where she's had to go to another site or take care of something that might have taken up her time? Keep in mind, it's still early August, and if the interim reviews were done in July/August, I would assume for both months. Reviews are something that take time and we have them going on around here too. We tend to do them by department so that HR isn't bogged down with 200+ reviews to log into the system for merit raises and so forth. Granted this isn't the case, but I'm sure she has her own personal paperwork to do, notes to input, etc. for her own use.
As for the e-mail, perhaps something worded along the lines of "I know we've missed our last 3 meetings and I would like to get together with you to make sure I'm on the right track. I'd live to discuss X with you and see how to handle it (that is if there is something going on that you need her input on)." And suggest a date that will work for you.