Covering the general science syllabus in 3 months is tough, but not impossible. If I were you, I would take these steps-

• Since I don’t have much time, first I would concise the syllabus by sorting what’s important and what’s not.
• I would collect all the necessary reference books or notes. I would also collect online sources such as youtube playlists regarding topics.
• I would make a study plan that outlines where to get started and what to read after what.
• I would spend the first 3 weeks understanding the basics , the next 5 weeks doing problems and the last 4 weeks doing revision.

Most importantly, I would try to utilize every bit of time I get, but remember, having proper sleep and diet is also part of the preparation.

I hope this helps and please feel free to ask if you have anymore questions.
Thank you and best of luck for your exam.