Good luck on the ISA. If you don't do too well, you can retake them in Yr11 so it's not the end of the world.
You only need 1 ISA for each subject - so for Core and Additional and the Extension (unit 3) they use the best ISA that you achieved for each subject. So if you got 30/40 as your best for Chemistry then they would use that mark for Core, Additonal and Extension if you were to do it.
I feel sorry for you Ramadullah - with the sort of awful responses you are having to put up with on this thread...
Good luck with your ISA - just make sure you revise everything and familiarise yourself with the questions - I recommend you have a go at the specimen papers, they are always very good practise and can be found on the exam board's website.
Once again, good luck!