While it's difficult for me to say whether 120 hours over 3 months using the books you mentioned will work for you, I will offer up how I did this without any formal training courses.
Regarding the study materials I used...
- PMBOK - I had the PMBOK at arms length but only picked it up when I had studied the info in a more 'friendly' manner first. In my opinion the PMBOK is a reference guide that is great for formulas, jargon, inputs/outputs etc.. It's not bedtime material...but is essential
- PMP for Dummies - Some folks like this series and others loathe them. I found that it explained the majority of information in a simple manner and provided the information which can then be enhanced upon by referencing the PMBOK and/or reading the same section in the other book I used
- Preparing for the PMP (Michael W. Newell) - I found this a very simple and easy to read book. It's small size and convenience fitted in with exactly what I wanted. As mentioned above, I would read some of this and then cross reference with the Dummies guide and (sometimes), the PMBOK
- Rita Mulcahey's flash cards on audio CD (no Rita Books). I just popped this into the car for about 10 days before taking the exam to help drill home some key principles, formulas, terminology etc...
- Also in the last couple of weeks I took some online sample exams. I actually failed 2 out of the 3 I took and then passed first time with an 87% score. So what I think they did more than anything was to force me to read the questions in detail and think before answering
I did not set a time limit for myself but instead just tried to read a little bit each week - and actually it turned out to be about 4 months. I might have also had a little jump start as I was already a practicing PM and held the CompTIA Project+ certification
One thing I did do after certification was to work with the HR department within my employer at the time, to establish a PMP study group which proved to be very succesful. If there are any study groups near where you work or live you might want to look into that also as it helps keep you on track and you can collaborate with other members on the hows and whys of different topics.