Hi Megha1412.
There is absolutely no reason to be "nervous" because your third practice test score was lower than your second by 30 points. People's scores fluctuate that much all the time, and you have just begun your preparation. So, you have knowledge and skill gaps that questions on one test may reveal, while another test may include more questions that you are ready to answer.
Regarding whether you can go from the low 600's to 750, you can for sure. At the same time, you have to realize that increasing one's GMAT score can take a fair amount of work. Achieving a 100+ point increase generally is not a matter of reviewing a few things and answering some practice questions. It's likely that you'll have to do some pretty substantial work to get your quant skills to where they have to be, for one thing.
One advantage that you do have is your strong verbal skills, as it tends to be easier to drive up one's quant score than one's verbal score. So, starting off with a foundation of relatively strong verbal skills can be helpful.
Regarding the time crunch in quant, it's common for people starting their GMAT preparation to have trouble finishing the quant section on time. At the same time, by developing familiarity and skill, you will speed up in quant.
Overall, the best way to increase your GMAT score and get to your 750 goal is to work topic by topic, driving your score up point by point. So, for instance, in quant, you would start with one topic, such as Number Properties or Probability, become strong in that area, and then move onto the next area.
For example, if you were focusing on Number Properties, you would first develop as much conceptual knowledge about Number Properties as possible. In other words, your goal would be to completely understand properties of factorials, perfect squares, quadratic patterns, LCM, GCF, units digit patterns, divisibility, and remainders, to name a few concepts. After carefully reviewing the conceptual underpinnings of how to answer Number Properties questions, you would practice by answering 50 or more questions just from Number Properties. When you do dozens of questions of the same type one after the other, you learn just what it takes to get questions of that type correct consistently.
Then you would move on to the next quant topic.
When you are working on learning to answer questions of a particular type, start off taking your time, and then seek to speed up as you get more comfortable answering questions of that type. As you do such practice, do a thorough analysis of each question that you don't get right. If you get a question wrong, ask yourself why. Did you make a careless mistake? Did you apply a formula incorrectly? Was there a concept you did not understand in the question? By carefully analyzing your mistakes, you will be able to efficiently fix your weaknesses and completely master each topic.
Speed won't come through your forcing yourself to speed up. Speed will come from your gaining knowledge and developing skill.
For more on how to increase your quant score, you could read the following post.
How To Increase Your GMAT Quant ScoreYou would work on verbal in a similar way, going through each topic in turn, learning the foundational concepts used in answering questions of a particular type and then applying those concepts by carefully answering dozens of questions of that type until you get close to all of them correct and become confident that when you see a question of that type that you will answer it efficiently and correctly.
As you would when doing quant practice, when you answer verbal questions, take your time at first, seeking to CLEARLY DEFINE why wrong choices are wrong and correct answers are correct. You won't get to 750 using gimmicky strategies. You have to learn to see exactly what there is to see and use solid logic to arrive at correct answers.
You may also find the information in this post helpful. It includes not only pointers on how to learn to answer GMAT questions but also ideas that you can use to plan your preparation and make your preparation efficient and effective.
How to Score a 700+ on the GMAT — A Mini Guide for Success