I think that your GMAT scores are on the low end for the schools you are targeting. But your chances depend on a variety of factors including years of work experience, your profession, undergrad experience + gpa, etc. I don't believe that a high GMAT score alone will help you get into the school of your choice, but I do know that a low score (below the low end of your target school's range) will weaken your application in most cases.
Which practice tests have you been taking? I ask since consistently scoring 600 on Kaplan's practice exams is different than consistently scoring 600 on Princeton Review's. And yes, I do feel you are too late in the game to produce three quality applications and meet the October deadlines. Of course, this is just my opinion, but you and I are in similar situations. My practice scores were in the 600-650 range about 3 weeks ago, but I took the practice exams before I had an opportunity to throughly review the math and verbal concepts which are tested on the GMAT. I wanted to apply to three schools in the first round (mid-Oct deadlines), but found that I needed to improve my GMAT scores first before doing so. You need to check with your target schools to figure out what GMAT score is acceptable.
My advice for self-study: USE THE MANHATTAN GMAT GUIDES. Manhattan GMAT's study guides are the best in my opinion since they do a thorough review of most GMAT topics and explain a variety of strategies in plain english, but you need to spend some time going through them. I found that reviewing the guides and then working through the corresponding problems in the Official Guide version 11 is an efficient method of self-study. During my practice tests I often found myself staring at what seemed to be a complex or confusing math problem which looked as if it would require too much time, so I skipped it or took an educated guess. After going through the guides, many of these problems now seem easy or manageable. Examples of such problems include rates and overlapping sets. And no, I don't work for Manhattan GMAT, nor do I have any affiliation with them.
I wish somebody had told me about these guides earlier, but unfortunately most of the people I know who scored well on the GMAT told me they barely studied for their exam at the beginning of my studies(some people love to make themselves seem naturally smart at the expense of others). So taking one guy's advice, I picked up the GMAT 800 book by Kaplan and used that alone to study thinking it was enough. I reviewed the book, completed the problems and reviewed the explanations. While taking my first practice test I quickly learned that I needed to review more concepts.
I was going to take the test in early Sept, but pushed it back three weeks to give myself sufficient time to review the MGMAT guides. I am nearly finished reviewing the MGMAT guides and need time to wrap up the OG11 problems and take 2 or 3 practice tests. If I mess up the first time, I can re-take in Oct and still have 2 full months to complete three applications and apply during Round 2. Remember, the extra time you have during Thanksgiving and X-mas break will help a lot.
I definitely feel as if I have a better grasp of what is tested on the GMAT now. Going through the problems in the OG11 helped me to improve my timing and recognize the test maker's patterns and traps. Some say that the same questions from the Official Guide will/may reappear on the real exam. I don't know if this is true, but I'm not going to rely on this. I think it's most important to determine which concepts are intuitive to you (strengths) and which you need to learn so that they become intuitive to you. Rote memorization will only get you so far in this exam.
Also, I would use the Kaplan 800 book as you near the end of your studies. It's a great book for further review of advanced concepts. Good luck.
