As LUGO said one way or other one or two of the sections you should do it fast, and that way you have time to concentrate on spend more time on your hardest section, be it RC or CR or SC. SC is probably the easiest one to get it fast. That is why it is important to know the patterns in SC. I can tell you by practice you surely can increase the RC speed. But you need to really work on it. May be do like 3 passages each day and concentrate fully, mostly with the structure and flow of the passage. Once you get this you can always go back and find where to look and get the answer.
Reading passage for 2/3/4 minutes is impossible for me. Moreover, for SC, I do not get the overall meaning of the sentence if i read it first time. Some times it is very tough for me to understand actual intention of the sentence. Most of the time, I can not concentrate deeply to get the idea as well.
For two/three months' preparation, how can i improve my scoring ability?
You have around 5-8 minutes to read each passage of the GMAT and more than once as long as you work on CR and SC on about 1-2 minutes per question. There are 4 passages in the GMAT and only one of them is longer than usual. If you want to improve your score I suggest you concentrate on learning how quickly to answer both CR and SC, so that you have enough time to tackle your weakest part, i.e. RC.