I did OCR so I can't really say if this is true for the other exam boards but you get 9 test papers over the course of two years. The first three are the same as core science so P1,2 &3, C1,2&3. And B1,2&3. The second is additional science, so 4,5&6 for each and the last is further additional so 7 for each. The hardest exam by far is further additional. Just because it's one topic, don't let it fool you that there isn't a lot to learn. The revision guides that I bought did summary pages at the end of section and for core and additional they were both about 2 pages long, for further additional they were 5 pages long.
As for revision guides I would recommend CGP because I found them really useful. If you are doing triple I would recommend buying them for each subject rather than each exam (e.g buying the textbook for GCSE chemistry rather than core science) because they have more information that's really good for triple science.
As for topics, I don't know which exam board you do so I can't really say what topics you'll do.
This may come off a bit cringy but as someone who's already done these exams, don't put the work off until last minute, triple science takes a lot of hard work